Before DNA testing became prevalent, the FBI relied heavily on visual analysis of evidence, sometimes building cases on microscopic analysis of hair.
But in 2012, federal officials launched an investigation after the Washington Post reported that flawed forensic hair matches might have led to the convictions of hundreds of innocent people.
In a full report coming out Monday, the Innocence Project and the National Association of Criminal Defense Lawyers will say that from the 1970s through 1999, 25 of 29 FBI forensic examiners had errors in their testimony -- impacting 257 cases.
The report also says out of 18 executions, 12 of them had errors in either testimony or lab reports.
Innocence Project co-founder Peter Neufeld says this could mean hundreds of appeals nationwide.
"The first order of business, frankly, is the FBI and DOJ has to redouble their efforts to get the transcripts of the hundreds and hundreds of cases of people where FBI agents gave exaggerated testimony," said Neufeld.
The Justice Department issued a statement saying: "the Department and the FBI are committed to ensuring that affected defendants are notified of past errors and that justice is done in every instance."
"The question is why this was permitted to continue," said Connecticut Senator Richard Blumenthal. "And that's a question the FBI has to answer after an investigation, a systemic analysis of the root causes. Why did it happen?"
Not all the cases depend heavily on the flawed hair match analysis which will make it more difficult for some to appeal their cases.