Chain of Custody in Expert Witness Testimony

Foundation and the Expert's Role in Authentication

When evidence reaches trial, it must be authenticated—that is, established as what it purports to be. The spectrum of authentication testimony is broad, ranging from a witness who observed the item at the moment of collection to testimony establishing that items were taken, that custody has been accounted for throughout the period until trial, and that laboratory analysis has occurred reliably. [1] An expert witness is often the party best positioned to describe these custody intervals, particularly when the expert has examined the evidence, conducted testing, or supervised its handling in a laboratory or forensic environment.

The expert's testimony on chain of custody is not separate from the expert's core opinions. Rather, it forms the factual and evidentiary foundation upon which those opinions rest. [2] Under Federal Rule of Evidence 702, as amended effective December 2023, an expert's testimony is admissible only if it is based upon sufficient facts or data, reflects the reliable application of reliable principles and methods, and the expert has reliably applied those principles and methods to the facts of the case. [7] Chain of custody testimony directly addresses the first requirement—sufficiency of data—and implicates reliability across all three. If the foundation for the data itself is compromised, the opinion built upon it is at risk.

Identifying Custody and Recording Transfers

The building blocks of chain of custody testimony are elementary but demanding. Each person who handles evidence must be identified by the expert. [4] All periods of custody must be properly accounted for and recorded. [4] Documenting the chain of custody literally requires that each person who touches an item sign for its possession. [4] An expert witness, even if she did not conduct the initial collection, must be prepared to testify to these details from the case record—the written logs, receipt forms, photographic documentation, and the testimony of prior custodians.

A typical chain of custody checklist, which the expert should understand and be prepared to address, includes: the field location of the item with careful log entry and photograph; how the item was preserved in bagging or packaging to maintain evidentiary value, with complete identification tags and labels; and the identity of all persons in the chain of physical custody, with log entries and receipts as the item passes from one custodian to the next. [5] The expert may not have participated in every step, but she should have reviewed the documentation and be able to vouch for its completeness and accuracy as the predicate for her own testing or analysis.

The Consequences of Incomplete Documentation

Failure to establish identity, authenticity, legal integrity, and a complete chain of custody for any item that passes through a laboratory may result in exclusion of the evidence or a limiting instruction to the jury regarding how to weigh the testimony. [4] This is not a mere formality. The court recognizes that breaks, gaps, or inadequately documented transfers create reasonable doubt about whether the item tested is the item seized, whether it has been altered, or whether substitution has occurred. The expert cannot testify around these gaps; they go to the weight and reliability of the entire analysis.

An expert examining evidence should be prepared to state, on direct examination, what documentation of custody she reviewed, what it shows, what she observed about the condition and integrity of the evidence when she received it, and what steps she took to preserve and document it while it was in her custody. This testimony should be grounded in specific, contemporaneous records—not memory or assumption.

Cross-Examination and the Disclosure Requirement

On cross-examination, an expert may be required to disclose the underlying facts or data upon which her testimony is based. [6] Opposing counsel will press on gaps in custody records, ask why certain transfers were not witnessed, explore whether anyone had access to the evidence, and question whether the expert took independent steps to verify the chain before beginning analysis.

To withstand this scrutiny, the expert must be able to point to documentary evidence—a receipt signed by the prior custodian, a laboratory entry log, photographic documentation of the seal or condition of the evidence. When challenged on chain of custody, the proponent of the expert testimony will ordinarily offer factual foundation keyed to specific testimony. [9] The court, preferably after an evidentiary hearing, will determine whether that foundation is sufficient. [9] The expert's credibility is at stake; she is expected to have done her own due diligence to confirm that the item she tested is the item that was seized.

Gatekeeping and Reliability Under Rule 702

Expert testimony should not be presumed admissible on its face. [11] An attorney seeking to have an expert's testimony admitted must prove, according to a preponderance of the evidence standard, that the expert's methodology and principles are reliable, and that the expert reliably applied those methodology and principles to the case. [11] This gatekeeping function applies to all expert testimony, not only scientific testimony. [11]

Chain of custody testimony is subject to this same scrutiny. The expert must demonstrate that her methods for verifying custody are sound, that she applied those methods carefully to the case at hand, and that her conclusions about the integrity of the evidence are supported by the record. Rule 702 requires that the means by which testimony was derived be reliable, requiring three showings: (i) that the method or principle used by the witness is reliable; (ii) that the witness used sufficient facts and data as required by the method or principle; and (iii) that the witness properly applied the method or principle to the facts and data. [6]

For chain of custody, this translates into a requirement that the expert have a reliable procedure for accepting evidence, documenting condition, maintaining separation from other items, recording access, and memorializing the chain before release. It is not enough to say, "I received a hard drive"—the expert must describe what she checked, what records she reviewed, what seals or markings she observed, and how she satisfied herself that this drive is the one seized from the suspect's residence on a particular date.

Procedural Requirements and Timely Disclosure

Parties challenging an expert's chain of custody testimony or the foundation for it must give notice. Motions in limine and any notices of intent to contest foundation, chain of custody, or scientific analysis must be filed at or before the final pretrial conference. [8] Unless exhibits are unusually voluminous, any notice of intent to contest foundation, chain of custody, or scientific analysis must provide a brief item-by-item or exhibit-by-exhibit description of the good faith basis for any objection. [8] This requirement serves an important purpose: it gives the expert and the proponent of her testimony adequate time to respond, to supplement the record, or to elicit additional testimony that addresses the deficiency.

An expert should anticipate such challenges and prepare thoroughly. Before trial, she should review all custody documentation, identify any gaps or ambiguities, and be ready to explain them or to testify about the measures she took to confirm the integrity of the evidence despite incomplete records. Where records are genuinely incomplete, transparency is better than improvisation.

Practice Points for the Practitioner

When preparing an expert witness to testify about chain of custody, ensure that she has reviewed and can reference the actual documents—not summaries or paraphrases. Walk through the timeline: who collected the evidence, when, under what conditions, what was its condition, who transferred it and when, what documentation exists for each transfer, what condition was it in when the expert received it, what did she do to verify its integrity, what access controls did she maintain, what documentation did she create. The testimony should be detailed and documentary. Vague references to "standard procedures" or "I assumed it was handled correctly" are vulnerable on cross-examination.

Opposing counsel will test the expert's knowledge of the chain. She may be asked about periods of custody that she did not personally supervise, about gaps in documentation, about whether the evidence could have been altered or substituted, about how she ruled out tampering. The more specific and document-grounded the expert's testimony, the more credible her responses will be. A careful expert will also note, and disclose, anything unusual about the custody or condition of the evidence that came to her attention—a broken seal, a gap in the log, a missing receipt. Addressing such issues forthrightly, with explanation, is far stronger than having opposing counsel surprise the expert with them on cross.

Chain of custody testimony is foundational. It undergirds the reliability of all that follows. Courts and juries understand this intuitively. An expert who takes custody seriously, documents it meticulously, and testifies to it candidly earns the credibility that her substantive opinions require.