Evidence collection and chain of custody

In short

Evidence collection is the disciplined recovery, documentation, packaging, sealing and preservation of physical, biological and digital material so that what reaches the laboratory and the court is demonstrably the same material that was at the scene, unchanged. Chain of custody is the unbroken written record of every person who held that material, when, and why — it converts an object into admissible evidence. Under the new criminal laws (BNSS 2023, BSA 2023, BNS 2023) collection is now statutorily tied to videography and, for serious offences, to a forensic expert's visit to the scene. It is a guaranteed question in B.Sc/M.Sc Forensic Science papers, in NFSU/DFSS and state FSL recruitment, in SI/CBI/NIA technical interviews, and in defence cross-examination — most forensic acquittals in India turn on custody defects, not on bad chemistry.

Explained

What counts as evidence, and how it is classified

• Physical (real) evidence: any object or trace — blood, hair, glass, bullet, soil, drug, document, hard disk. Documentary evidence is writing/records; oral evidence is testimony (BSA 2023 recognises electronic records as documents).
• Direct vs circumstantial: direct proves a fact by itself (eyewitness); physical evidence is almost always circumstantial — it links, it rarely identifies alone.
• Class characteristics: features shared by a group (blood group B, shoe size 9, "Nike" sole pattern) — give exclusion and narrowing.
• Individual characteristics: features unique to one source (fingerprint minutiae, striations on a fired bullet, DNA profile, random wear cuts on a sole) — give identification.
• Transient evidence: lost quickly — odour, temperature, wet stain, a running process in RAM. Collect first.
• Conditional evidence: produced by an event and fixed only by recording — door locked/unlocked, lights on, position of body, vehicle ignition state.
• Pattern evidence: bloodstain patterns, glass fracture, tyre marks, fire burn patterns — recorded photographically, not "collected".
• Transfer/trace evidence: material moved between persons, objects and places (fibres, soil, GSR), the basis of linkage.
• Associative evidence: personal items that tie a person to a place (ID card, SIM, bank slip).
• Reference (known) sample: from a known source for comparison — buccal swab of suspect, hair standard, glass from the broken window.
• Control/substrate sample: unstained portion of the same substrate, to prove the substrate itself did not cause the result.
• Blank: clean swab/solvent carried through the process to prove no contamination.

Collection, packaging and preservation — technique by material

• General rules: photograph *in situ* with a scale and a number marker before touching; one item, one container, one label; dry evidence goes in breathable paper (bags, envelopes, bindles), volatiles in airtight metal/glass; never seal anything damp in polythene (mould and bacteria destroy DNA within days).
• Tools and methods: gloved hand and forceps for large items; swabbing (double-swab: first swab moistened with sterile water/saline, second dry, same area); tape lifting for fibres and residues; scraping for dried blood and paint; vacuum sweeping as a last resort (unselective); cutting out a section of the substrate when the stain cannot be removed; casting with dental stone for footwear/tyre impressions and silicone (Mikrosil-type) for toolmarks; powder-and-lift or hinged lifters for latent prints; chemical/fuming methods for prints on difficult surfaces.
• Liquid blood: in EDTA vacutainer (purple cap) for DNA/grouping; with sodium fluoride + potassium oxalate (grey cap) for alcohol/toxicology; refrigerate 2–8 °C, do not freeze whole blood meant for serology.
• Dried bloodstain: scrape into a paper bindle, or lift on a lightly moistened swab, air-dry at room temperature away from sunlight and heat, then paper packet.
• Semen/saliva/vaginal swabs: air-dried swabs in paper; sexual assault samples follow the sealed kit (SAFE/SAEC kit) and MoHFW *Guidelines and Protocols: Medico-legal Care for Survivors of Sexual Violence*; medical examination report is to be forwarded to the investigating officer within seven days (BNSS provision corresponding to old Section 164A CrPC).
• Hair and fibres: paper druggist's fold (bindle) inside an envelope; never loose in an envelope corner; pull 25–50 reference hairs with roots for DNA.
• Fire debris / arson: unlined metal can or nylon (not polythene) bag, filled to about two-thirds, airtight; passive headspace with activated charcoal strip at the lab; collect a comparison sample of unburnt substrate.
• Firearms and ammunition: unload and record chamber condition, cable-tie action open, rigid box; do not clean the barrel; GSR from hands/clothing on adhesive SEM stubs within hours (washing destroys it); dermal nitrate/paraffin tests are obsolete and unreliable.
• Viscera in suspected poisoning: stomach with contents, upper small intestine with contents, about 500 g liver, one kidney (half of each), and blood and urine; preserved in saturated common-salt solution in separate clean glass jars with a jar of the preservative itself as a control; a second set in rectified spirit is sent where volatile poisons are not suspected (spirit is never used when alcohol is in question).
• Documents: handle by edges in transparent/paper folders, never fold along existing creases, no staples or pins; collect standards — requested and natural/admitted writing.
• Digital evidence: follow the order of volatility (RAM and network state, then disk); photograph screen, isolate the phone (Faraday bag or airplane mode plus charger), use a write blocker, take a bit-stream image, and record hash values (MD5/SHA-1/SHA-256) before and after; the hash *is* the integrity chain. Produce the certificate under BSA 2023 Section 63 (the successor to Section 65B of the 1872 Act) with the seizure.
• Storage temperatures (typical lab practice): room temperature dry for dried stains and hair; 2–8 °C for liquid blood, urine, viscera awaiting analysis; −20 °C or below for long-term DNA extracts and tissue; bones and teeth dry at room temperature.

Documentation at the scene

• Notes: continuous, timed, in ink, no erasures; who collected, exact location, description, condition, container and seal number.
• Photography: overall, mid-range (item in relation to a fixed feature) and close-up with and without a scale; 1:1 for prints and impressions; maintain a photo log.
• Sketch and measurements: rough sketch at the scene with measurements from two fixed points, fair sketch later, to scale, with legend and north arrow.
• Videography: under BNSS 2023 search and seizure must be audio-video recorded, preferably on a mobile phone, and the recording forwarded to the District/Judicial Magistrate without delay; the MHA/NIC eSakshya app is being used for this, with upload to the Inter-operable Criminal Justice System.
• Seizure memo / panchnama: list of articles with description, weight/quantity, seal details, signed by the seizing officer, the accused/occupant where present, and two independent respectable witnesses of the locality (the BNSS provision carried over from Section 100 CrPC).
• Evidence labels: case/FIR number, item number, description, place and time of seizure, name and signature of collector.

Sealing, forwarding and the chain of custody

1. Seal at the point of seizure. Pack, wrap in cloth where required, stitch/tie, and seal with lac or a tamper-evident seal bearing a specimen seal impression; the specimen impression is sent separately to the court/lab so tampering can be detected, and the seal is handed to a person other than the investigating officer.
2. Malkhana. Entry in the malkhana (store-room) register with item, seal condition, rack/box number, and the signature of the *moharrir*/store in-charge; every subsequent movement re-entered.
3. Forwarding to the FSL. A forwarding letter/requisition from the investigating officer or through the Magistrate states the case, the items, the examination sought and the questions to be answered; carried by a named constable or dispatched by a recorded route.
4. Laboratory receipt. Condition of seal verified and recorded on receipt; internal custody register tracks movement between divisions and analysts; exhibits re-sealed with the laboratory seal after examination and returned.
5. Court. Exhibit produced, seal shown intact, the custody record proved by each holder; the expert's report may be admitted without personally examining the expert under the BNSS provision on reports of government scientific experts (Section 329 BNSS, successor to Section 293 CrPC), but the court can summon the expert.
6. Chain of custody defined: the chronological, documented record of the seizure, custody, control, transfer, analysis and disposition of evidence — every entry answers who, what, when, where, from whom, to whom, and why. A link with no document is a break; a break is not automatically fatal but shifts the burden to the prosecution to explain it.
7. Minimum fields of a CoC form: case number, item number and description, date/time and place of collection, collector, seal number, each transfer with date, time, both signatures and purpose, storage conditions, and final disposal.

Indian law, agencies and standards

• BNSS 2023, Section 176(3): for offences punishable with seven years or more, a forensic expert shall visit the scene, collect forensic evidence and videograph the process; where the State lacks the facility it may use another State's, with a five-year window notified for full roll-out.
• BNSS 2023: audio-video recording of search and seizure and its transmission to the Magistrate; search by a police officer with reasons recorded (successor to Section 165 CrPC); summons to produce a document or electronic communication/device (successor to Section 91); Magistrate's order for specimen signature, handwriting, finger impressions and voice samples, including from a person not arrested (Section 349, widening old Section 311A); medical examination of the accused and of the victim in sexual offences.
• BSA 2023 (Bharatiya Sakshya Adhiniyam): electronic records are documents and primary evidence in defined cases; Section 63 certificate for electronic records; Section 39 on expert opinion (successor to Sections 45 and 45A, expressly including the examiner of electronic evidence); the discovery exception to confessions to police (old Section 27 of the 1872 Act, carried into BSA Section 23) — the basis on which recoveries at the accused's pointing out are proved.
• Criminal Procedure (Identification) Act, 2022: replaced the Identification of Prisoners Act, 1920; permits collection of finger, palm and foot impressions, photographs, iris and retina scans, physical and biological samples, and signature/handwriting of convicts, arrested persons and certain others, stored centrally by NCRB (statutorily for 75 years).
• NDPS Act, Section 52A read with the Supreme Court's directions in *Union of India v. Mohanlal* (2016): samples are drawn, and the inventory certified, in the presence of a Magistrate; departmental Standing Orders fix the representative quantity — commonly 24 g for opium, ganja and charas and 5 g for other drugs, with packages bunched in lots (verify the current Standing Order, as quantities and bunching rules have been revised).
• Agencies: Directorate of Forensic Science Services (DFSS) under MHA with the Central FSLs; State and Regional FSLs and mobile forensic vans; BPR&D manuals and SOPs on crime-scene management; National Forensic Sciences University, Gandhinagar (institution of national importance, 2020); NCRB for fingerprint (NAFIS) and records.
• Quality standards: ISO/IEC 17025:2017 accreditation by NABL for testing laboratories (and ISO/IEC 17020 for inspection bodies); ISO 18385 "forensic-grade" DNA-free consumables; documented SOPs, calibration records, proficiency testing, and analyst competency files — all of which the defence may demand.
• DNA-specific law: India has no dedicated DNA legislation — the DNA Technology (Use and Application) Regulation Bill was withdrawn in 2023 — so DNA evidence is governed by the general evidence law plus laboratory SOPs.
• Leading case law: *Union of India v. Mohanlal* (2016) on drug sampling and disposal; *Anil v. State of Maharashtra* (2014) on documenting DNA sample handling; *Dharam Deo Yadav v. State of U.P.* (2014) accepting DNA where custody was proved; *Rahul v. State (NCT of Delhi)* (2023), where sloppy collection and unexplained custody gaps contributed to acquittal in a capital case; *Arjun Panditrao Khotkar v. Kailash Gorantyal* (2020) on the mandatory certificate for electronic records (now BSA Section 63).

Contamination, integrity and limitations

• Contamination control: full PPE (gloves, mask, cap, shoe covers, coverall); change gloves between items; single-use or decontaminated tools; no eating, smoking, drinking or phone use in the zone; restrict entry through a single path and keep an entry log; collect elimination prints and DNA from attending officers and scene personnel.
• Degradation: heat, humidity, sunlight, bacteria and freeze–thaw cycles destroy DNA and alter drug and alcohol concentrations (post-mortem alcohol can be *generated* microbially if fluoride is omitted).
• Limitations: trace evidence shows association, not act or intent; secondary and tertiary transfer can place a fibre or DNA where the person never was; "absence of evidence" is weak proof of absence; a perfect analytical result on an exhibit with a broken chain is worth little in court.
• Legal consequence of a break: the exhibit's identity and integrity become doubtful, the expert's report loses its evidentiary link to the crime, and the benefit of doubt goes to the accused.

Key points

• Chain of custody – documented, unbroken record of every holder of an exhibit from seizure to court; answers who, what, when, where, why.
• Locard-based linkage – every contact leaves a trace, so collection aims at transferred material, not just obvious objects.
• Transient first – odour, temperature, wet stains and volatile/RAM data are collected or recorded before anything stable.
• One item, one container – prevents cross-transfer; each separately sealed and labelled.
• Paper for biological, airtight for volatile – polythene traps moisture (mould, DNA loss); fire debris needs a vapour-tight can or nylon bag.
• Air-dry, never heat-dry – room temperature, out of sunlight, before packaging any damp item.
• Control and reference samples – substrate control, blank and known standard must accompany the questioned sample or comparison is unsound.
• Specimen seal impression – sent separately from the exhibit so tampering with the seal is detectable.
• Panchnama/seizure memo – signed by the officer, the occupant and two independent local witnesses.
• BNSS 176(3) – forensic expert visit and videography compulsory for offences punishable with seven years or more.
• BNSS search videography – audio-video recording of search and seizure, forwarded to the Magistrate (eSakshya in practice).
• BSA Section 63 certificate – required to prove an electronic record; hash values evidence integrity.
• BNSS Section 349 – Magistrate may order specimen handwriting, finger impressions and voice samples.
• BNSS Section 329 – report of a government scientific expert admissible without calling the expert, subject to the court summoning them.
• Criminal Procedure (Identification) Act, 2022 – legal basis for taking biological samples, iris/retina scans and impressions.
• NDPS Section 52A – sampling and inventory certified before a Magistrate; representative quantity per Standing Order.
• ISO/IEC 17025:2017 + NABL – the quality framework a defence lawyer will test; ISO 18385 for DNA-free consumables.
• Breaks are explainable, not automatically fatal – but the prosecution must account for every gap, and courts have acquitted where it could not.

Formulas and examples

Example 1 — preservative for a blood sample (toxicology). 10 mL of blood is drawn for blood-alcohol estimation; practice is 1 % w/v sodium fluoride (preservative) and 0.3 % w/v potassium oxalate (anticoagulant).

• NaF = 1 × 10 × 10 = 100 mg; potassium oxalate = 0.3 × 10 × 10 = 30 mg.
• The vial is filled nearly full, capped, labelled, refrigerated at 2–8 °C. Omitting fluoride can allow microbial production of ethanol — a defence point the chemist cannot repair later.

Example 2 — NDPS representative sampling. 36 identical gunny bags of ganja, 1 kg each, are seized; lot size 40 for ganja and charas, sample quantity 24 g.

• N = ceil(36 / 40) = 1 sample (all 36 identical bags bunch into a single lot).
• Total drawn = 1 × 24 g = 24 g, taken in equal parts from every bag and mixed into one composite sample; each sample is drawn in duplicate (one for the lab, one in reserve), i.e. 2 sealed packets of 24 g = 48 g.
• Drawing and the inventory are certified before a Magistrate under Section 52A NDPS; the balance 36 kg − 0.048 kg = 35.952 kg goes to the godown for certified disposal. (Confirm the current Standing Order: quantities and bunching have been revised over time.)

Example 3 — dental stone cast of footwear impressions. Three impressions in soil, 1 kg of dental stone budgeted per cast, r = 0.30.

• Water = 3 × 1,000 × 0.30 = 900 mL for 3 kg of stone.
• Procedure: photograph with scale first → frame the impression → pour through a baffle, never directly onto detail → mark case number, date, north arrow and collector's initials on the back while setting (about 20–30 minutes) → lift after 30 minutes, clean only after 24–48 hours of curing.

Example 4 — reading a chain-of-custody record.

• Custody entries = 4, transfers = 3. Elapsed time, seizure to lab receipt = 03-11 10:40 to 07-11 11:00 = 96 hours 20 minutes (4 days 20 minutes).
• The 3 h 40 min scene-to-station leg and the 4-day malkhana leg are both covered by signed entries, so there is no break; had entry 3 been missing, the 92 h 40 min (3 days 20 h 40 min) from 14:20 on 3 November to the lab handover at 11:00 on 7 November would be an unexplained gap and the defence would attack identity of the exhibit.

Example 5 — digital exhibit integrity. A 500 GB laptop drive is imaged through a write blocker at 100 MB/s.

• Size = 500 × 1,000 = 500,000 MB → time ≈ 500,000 ÷ 100 = 5,000 s = 1 hour 23 minutes (plus a verification pass).
• SHA-256 of the source and of the image are compared; each is 64 hex characters. Equal hashes prove bit-for-bit identity; a single changed bit changes the digest completely.
• Example entry: SHA-256 (source) = 3f1a…9c | SHA-256 (image) = 3f1a…9c → VERIFIED
• The seizure is accompanied by the BSA Section 63 certificate; analysis is done only on a working copy, never the original.

Where it is used

• State and Central FSLs and mobile forensic units (DFSS, CFSL Hyderabad/Kolkata/Chandigarh and State FSLs) — exhibit receipt, seal verification, internal custody registers, NABL-compliant records.
• Police investigation and prosecution — IOs, SHOs, malkhana in-charge, crime-scene units raised in every district to meet the BNSS 176(3) forensic-visit requirement; eSakshya videography and ICJS upload.
• Medico-legal practice — government hospitals and medical colleges handling autopsies, sexual-assault kits and injury cases; sample forwarding to FSL and reporting timelines.
• Private forensic and digital-forensic firms, banks and insurers — fraud investigations, questioned documents, cyber incidents, e-discovery where custody and hashing decide admissibility.
• Corporate and regulatory roles — internal investigations, drug-testing programmes, food and drug regulators, excise and NCB, where statutory sampling procedures mirror these rules.
• Job roles — Scientific Assistant/Scientific Officer in FSL, Crime Scene Manager, Forensic Investigator, Digital Forensic Analyst, Medico-legal Technician, Expert Witness; entry routes include state FSL recruitment, NFSU placements, CBI/NIA/NCB and private labs.

Interview questions

• Q: What exactly is chain of custody and why does a court insist on it? A: It is the documented, unbroken record of everyone who held an exhibit and when, with purpose and storage conditions; it proves that the item tested is the item seized and that it was not altered or substituted.
• Q: A break in the chain is found. Is the evidence automatically inadmissible? A: No — admissibility is not automatically lost, but the prosecution must explain the gap; if it cannot, the court may doubt the exhibit's identity and give the accused the benefit of doubt.
• Q: Why is biological evidence packed in paper rather than plastic? A: Paper breathes, so residual moisture escapes; plastic traps humidity and promotes bacterial and fungal growth that degrades DNA and alters serological results.
• Q: What does BNSS Section 176(3) change for a forensic scientist? A: For offences punishable with seven years or more, a forensic expert must visit the scene, collect the evidence and videograph the process, making scene-level forensic attendance a statutory duty rather than a request.
• Q: Why is the specimen seal impression sent separately from the exhibit? A: So the court or laboratory can compare the seal on the packet with an independently received impression; sending both together would let a tamperer replace both.
• Q: What is the difference between a reference sample and a control sample? A: A reference (known) sample comes from a known source for comparison with the questioned sample; a control sample is unstained substrate or a blank used to show the substrate or the process did not generate the result.
• Q: Explain the double-swab technique and when you would use it. A: A moistened swab rehydrates and lifts cellular material from a non-absorbent surface, followed immediately by a dry swab over the same area to recover the remaining moisture and cells — used for touch DNA, dried saliva and dried blood on hard surfaces.
• Q: How do you preserve the integrity of a seized mobile phone? A: Photograph the screen, isolate it from networks in a Faraday bag or by airplane mode with power maintained, do not browse it, seize chargers and SIM, and image it through a write blocker with hash verification plus a BSA Section 63 certificate.
• Q: Which preservatives are used for a blood sample for alcohol estimation, and why? A: Sodium fluoride as a preservative to stop microbial fermentation or loss of ethanol, with potassium oxalate as anticoagulant — typically 1 % and 0.3 % w/v respectively.
• Q: How are samples of a seized narcotic drawn lawfully? A: Representative samples are drawn and the inventory certified before a Magistrate under Section 52A NDPS, in the quantities and lot sizes fixed by the departmental Standing Order, with the proceedings documented and the bulk sent for certified disposal.
• Q: Two exhibits from the same case are packed in one envelope to save seals. What is wrong? A: It permits cross-transfer of trace material between exhibits, destroys the evidential value of any linkage found, and makes the packaging itself a source of contamination.
• Q: The expert's report is in but the analyst has left the country. Can the report still be used? A: Yes — the BNSS provision on reports of government scientific experts (Section 329) allows such a report to be used without examining the expert, though the court may summon them and may require the person who conducted the analysis to depose.

Common mistakes

• Treating documentation as paperwork — no photograph with scale, no timed notes, no signed transfer entries, then trying to reconstruct custody from memory in court.
• Packing damp biological evidence in polythene, or heat-drying it, and destroying the DNA before the lab sees it.
• Collecting the questioned sample without a reference standard, a substrate control or a blank, leaving the result uninterpretable.
• Using plastic for fire-debris, or spirit as a preservative for viscera in a suspected alcohol case.
• Changing gloves rarely, bagging multiple items together, and failing to take elimination prints/DNA from the officers who entered the scene.
• Quoting repealed provisions in exams — Sections 65B, 45, 27 of the Evidence Act or 164A, 165, 293, 311A CrPC — instead of the BSA 2023 and BNSS 2023 equivalents, and forgetting that the Identification of Prisoners Act, 1920 is replaced by the 2022 Act.
• Saying "the DNA matched, so the case is proved" without establishing how the sample was sealed, stored and transported — the point on which Indian courts most often reject forensic evidence.

Related topics

Forensic serology and DNA profiling · Fingerprints and other pattern evidence · Forensic toxicology and viscera analysis · Questioned documents and handwriting examination · Digital and cyber forensics · Expert testimony and courtroom procedure