An unprecedented federal review of old criminal cases has uncovered as many as 27 death penalty convictions in which FBI forensic experts may have mistakenly linked defendants to crimes with exaggerated scientific testimony.

At issue is a once-widespread practice by which some FBI experts exaggerated the significance of “matches” drawn from microscopic analysis of hair found at crime scenes. Since at least the 1970s, written FBI Laboratory reports typically stated that a hair association could not be used as positive identification. However, on the witness stand, several agents went beyond the science and testified that their hair analysis was a near-certain match.

It is not known how many of the cases involve errors, how many led to wrongful convictions or how many mistakes may now jeopardise valid convictions. Those questions will be explored as the review continues. But it has already led to an 11th-hour stay of execution in Mississippi in May, after the Justice Department acknowledged flaws in forensic testimony by the FBI that helped convict Willie Jerome Manning of the 1992 murders of two university students. The 44-year-old had been hours away from receiving a lethal injection. Federal officials have offered to retest the DNA in the case.

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The number of other cases under review places the examination firmly at the heart of the debate about the death penalty. The death row cases are among the first 120 convictions identified as potentially problematic among more than 21,700 FBI Laboratory files being examined. The review was announced last July by the FBI and the Justice Department, in consultation with the Innocence Project and the National Association of Criminal Defense Lawyers (NACDL).

The unusual collaboration came after it emerged last year that authorities had known for years that flawed forensic work by FBI hair examiners may have led to convictions of potentially innocent people, but officials had not aggressively investigated problems or notified defendants.

The new review listed examples of scientifically invalid testimony, including claiming to associate a hair with a single person “to the exclusion of all others”, or to suggest a probability for such a match from past casework.

Whatever the review’s findings, the initiative is pushing state and local labs to take similar measures. Last week, the Texas Forensic Science Commission directed all labs under its jurisdiction to take the first step to scrutinise hair cases, in a state that has executed more defendants than any other since 1982. Separately, FBI officials said their intention is to review and disclose problems in capital cases even after a defendant has been executed.

“We didn’t do this to be a model for anyone – other than when there’s a problem, you have to face it, and you have to figure how to fix it, move forward and make sure it doesn’t happen again,” FBI general counsel Andrew Weissmann said. “That tone and approach is set from the very top of this building,” he said, referring to FBI director Robert S Mueller. David Christian “Chris” Hassell, director of the FBI Laboratory, said the review will be used to improve lab training, testimony, audit systems and research, as it has done when previous mistakes were uncovered. The lab overhauled scientific practices when whistleblowers revealed problems in 1996, and again after three highly skilled FBI fingerprint experts declared that Oregon lawyer Brandon Mayfield’s fingerprint matched a partial print found on a bag in Madrid that contained explosive detonators in 2004, during the investigation into the train bombings which killed 191 people. US officials had called the match “absolutely incontrovertible” and Mayfield was taken into custody, but the FBI later admitted it had got it wrong, and the print actually belonged to Ouhnane Daoud, an Algerian.

“One of the things good scientists do is question their assumptions. No matter what the field, what the discipline, those questions should be up for debate,” Hassell said. “That’s as true in forensics as anything else.”

Advocates for defendants and the wrongly convicted have called the review a watershed moment in police and prosecutorial agencies’ willingness to reopen old cases because of scientific errors uncovered by DNA testing. Peter J Neufeld, co-founder of the Innocence Project, which supports inmates who seek exoneration through DNA testing, applauded the FBI, calling the review historic and a “major step forward to improve the criminal justice system and the rigour of forensic science in the United States.”

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Norman L Reimer, executive director of the NACDL, also praised the effort, predicting that it would have “an enormous impact on the states” and calling on the defence bar to represent impoverished convicts. “That’s going to be a very big job as this unfolds,” said Reimer.

Unlike DNA analysis, there is no accepted research on how often hair from different people may appear the same. The federal inquiry came after the Public Defender Service helped exonerate three men from Washington DC through DNA testing that showed that three FBI hair examiners contributed to their wrongful convictions for rape or murder in the early 1980s.

The response has been notable for the department and the FBI, which in the past has been accused of overprotecting its agents. Twice since 1996, authorities conducted case reviews largely in secret after the scientific integrity of the FBI Lab was faulted.

Weissmann said that although earlier reviews lawfully gave prosecutors discretion to decide when to turn over potentially exculpatory material to the defence, greater transparency will “lessen scepticism” about the government’s motives. It also will be cheaper and more effective because private parties can help track down old cases.

The review terms could have wide repercussions. The FBI is examining more than 21,000 federal and state cases referred to the FBI Lab’s hair unit from 1982 through 1999 – by which time DNA testing of hair was routine – and the bureau has asked for help in finding cases before lab files were computerised in 1985.

Of 15,000 files reviewed to date, the FBI said a hair association was declared in about 2,100 cases. Investigators have contacted police and prosecutors in more than 1,200 of those cases to find out whether hair evidence was used in a conviction, in which case trial transcripts will be sought. However, 400 of those cases have been closed because prosecutors did not respond. While the FBI employed 27 hair examiners during the period under review, FBI officials confirmed for the first time this week that records indicate that about 500 people attended hair comparison classes given by FBI examiners from 1979 to 2009. Nearly all of them came from state and local labs. State and local prosecutors handle more than 95 per cent of violent crimes.

In April, the accreditation arm of the American Society of Crime Laboratory Directors declined to order state and local labs to conduct reviews, but issued a public notice recommending that each laboratory evaluate the impact of improper statements on past convictions. FBI Lab officials say they have not been contacted by other labs about their review or who completed the FBI classes.