The FBI has admitted "errors" in evidence provided by its forensics laboratory to US courts to help secure convictions, including in death penalty cases, over more than 20 years.
A report by the Office of the Inspector General (OIG) noted "irregularities" in the hair analysis unit.
More detail on the cases affected is expected later from campaign groups.
Flawed forensics were used in at least 60 capital punishment cases, the OIG report found.
Fourteen defendants were either executed or died in prison, says the Washington Post, which first reported the story at the weekend.
The review of cases was prompted by the Post's 2012 story that three men were wrongly placed at the scene of violent crimes by the unit's hair analysts, raising the possibility of hundreds of unsafe convictions.
In a statement, the FBI admitted "errors made in statements by FBI examiners regarding microscopic hair analysis in the context of testimony or laboratory reports".
It added: "Such statements are no longer being made by the FBI."
The statement added the FBI was "committed to ensuring that affected defendants are notified of past errors and that justice is done in every instance".
The OIG's report criticised "the use of scientifically unsupportable analysis and overstated testimony by FBI lab examiners in criminal prosecutions".
The Washington Post reported that of 28 examiners within the microscopic hair comparison unit, 26 overstated forensic matches in ways that favoured prosecutors in more than 95% percent of the 268 trials reviewed so far.
The paper cited data compiled by the National Association of Criminal Defence Lawyers and the Innocence Project, which seeks to free inmates convicted on the basis of faulty evidence.
The two groups are expected to release a report later providing more information on the errors which could lead to hundreds of appeals.