Much of this work looks like arithmetic, which creates a trap no other expert discipline has: whether the witness needed to be disclosed as an expert at all is genuinely unsettled.
Start a conversation with the Forensic Concierge, already scoped to investigative testimony. Pick a starting point, or describe the matter directly.
Three separate constraints govern what a financial investigator may tell a factfinder, and they are enforced by different mechanisms at different stages. The first is whether the testimony is lay or expert, which decides whether a Rule 26 report was ever required — and on which federal courts have reached opposite answers within a year of each other. The second is the vehicle: most tracing schedules reach a jury as Rule 1006 summaries, which carry their own discipline and their own limits on inference. The third is the sentence itself. The legal-conclusion bar, the mental-state rules and the profession’s own standards converge on a single prohibition, and the expert who reaches past it has given opposing counsel a motion to file.
Six limits on investigative testimony, each of which operates independently of the others.
Rule 701(c) bars lay opinion resting on specialized knowledge. In United States v. Fenner (7th Cir. 2025) an FBI forensic accountant’s tracing across tens of thousands of pages was lay testimony that ‘did not require expertise beyond the ken of a lay person’. In United States v. Joseph, 108 F.4th 1273 (10th Cir. 2024), using a certified fraud examiner as a lay witness was error because ‘he had to choose between multiple possible methodologies for tracing bank funds’.
The rule permits a summary, chart or calculation proving the content of voluminous admissible writings that cannot conveniently be examined in court, provided the originals or duplicates are made available. Summaries remain subject to Rule 403 and the hearsay rules, and are properly rejected if inaccurate or argumentative.
In United States v. Scop, 846 F.2d 135 (2d Cir. 1988), an investigator testified in language drawn from the charged statutes. The court held those statements embodying legal conclusions exceeded the permissible scope of opinion testimony and reversed all but the false-declaration convictions.
Rule 704(a) permits an opinion embracing an ultimate issue; Rule 704(b) bars expert opinion on a criminal defendant’s mental state as an element. Diaz v. United States, 602 U.S. 526 (2024) held group-based testimony about what most people in a category know is not an opinion about the defendant.
Rule 704(b) is criminal-only, so civil practitioners often assume no bar exists. The In re Rezulin line, 309 F. Supp. 2d 531 (S.D.N.Y. 2004), bars expert opinion on the intent, motives or states of mind of corporations under Rules 702 and 403.
Rule 26(a)(2)(B) report failures are enforced through Rule 37(c)(1), which is self-executing: undisclosed opinions cannot be used unless the failure was substantially justified or harmless. Conclusory reports that omit the how and why are the recurring defect.
How testimony is scoped so that it survives.
These are procedural questions with substantive consequences.
That is the workable test the two 2024–25 appellate decisions produce between them. Fenner treated a large volume of simple maths as lay testimony. Joseph held that selecting among competing tracing methodologies is specialized knowledge reserved for experts. Getting it wrong means no Rule 26 report was served for a witness who needed one.
It depends on what the witness had to decide, and the federal authority is genuinely divided. The Seventh Circuit in United States v. Fenner (2025) affirmed lay treatment of an FBI forensic accountant who traced money across tens of thousands of pages of bank records, reasoning that while the volume of maths was large the method was simple and the work required attention to detail rather than expertise beyond the ken of a lay person. The Tenth Circuit in United States v. Joseph, 108 F.4th 1273 (2024), held it error to use a certified fraud examiner as a lay witness because he had to choose among multiple possible methodologies for tracing bank funds. The practical test: if your witness selected a tracing convention, disclose them.
An expert bound by these standards cannot, and the reasons stack. AICPA SSFS No. 1 ¶10 prohibits a member performing forensic services from opining on the ultimate conclusion of fraud, reserving it to the trier of fact, while permitting opinions on whether evidence is consistent with certain elements. ACFE Code §V.B.2 bars any opinion on the legal guilt or innocence of any person or party. Independently, Scop makes an opinion phrased in the statute’s vocabulary a legal conclusion. But do not flatten this: the ACFE’s own guidance permits an examiner to conclude that a person misappropriated cash, misrepresented a transaction or concealed funds, and to conclude that each statutory element is satisfied. The line is conduct against legal guilt.
Narrower and more useful than most people expect. In United States v. May, 131 F.4th 633 (8th Cir. 2025), an FBI forensic accountant testified to cash deposits exceeding $15,000 into an account in 2015 against under $500 in cash deposits the previous year. That is a pattern change in the records, offered as circumstantial corroboration. The witness did not testify that anyone took a payment, that anyone knew anything, or that any arrangement existed. The factfinder was left to draw the inference. That structure — describe the movement, describe the change, stop — is what keeps the testimony inside Rule 702(d), inside Rule 704, and inside the profession’s own standards at the same time.
Often not. A defective Rule 26(a)(2)(B) report met with Rule 37(c)(1) exclusion is self-executing, and practitioner accounts treat it as a more frequent way of losing an expert than a contested reliability challenge. Where Rule 702 is invoked, the most commonly cited grounds remain reliability — insufficient data, or a methodology not generally accepted — and relevance, and relevance catches opinions that are methodologically sound but not tied to the facts in issue. There is also a separate vehicle-level exposure: where testimony arrives as a Rule 1006 summary, the Seventh Circuit in Fenner noted that a summary witness may testify to what the government’s evidence shows but cannot offer implausible or speculative inferences, and should tread carefully when opining on intent. Counsel who prepare only a Daubert motion have aimed at the wrong rule. For general admissibility and expert-selection questions, our Economic Damages Institute covers that ground.
Describe the analysis and the forum. The Institute will help you see where investigative testimony is usually constrained, and by which rule.