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, U.S. officials said.

The review led to an 11th-hour stay of execution in Mississippi in May.

It is not known how many of the cases involve errors, how many led to wrongful convictions or how many mistakes may now jeopardize valid convictions. Those questions will be explored as the review continues.

The discovery of the more than two dozen capital cases promises that the examination could become a factor in the debate over the death penalty. Some opponents have long held that the execution of a person confirmed to be innocent would crystallize doubts about capital punishment. But if DNA or other testing confirms all convictions, it would strengthen proponents’ arguments that the system works.

FBI officials discussed the review’s scope as they prepare to disclose its first results later this summer. 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 came after The Washington Post reported 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.

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 for years went beyond the science and testified that their hair analysis was a near-certain match.

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 state or suggest a probability for such a match from past casework.

Whatever the findings of the review, the initiative is pushing state and local labs to take similar measures. For instance, the Texas Forensic Science Commission on Friday directed all labs under its jurisdiction to take the first step to scrutinize 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 Mueller III.

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 breakdowns were uncovered. The lab overhauled scientific practices when whistle-blowers revealed problems in 1996 and again after an FBI fingerprint misidentification in a high-profile 2003 terrorism case, he said.

“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 called the undertaking a watershed moment in police and prosecutorial agencies’ willingness to reopen old cases because of scientific errors uncovered by DNA testing.

Peter 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 rigor of forensic science in the U.S.”

Norman Reimer, executive director of the National Association of Criminal Defense Lawyers, also praised the effort, predicting that it would have “an enormous impact on the states” and calling on the defense bar to represent indigent convicts.

“That’s going to be a very big job as this unfolds,” said Reimer, whose group has spent 1,500 hours identifying cases for the second round of review.

Under terms finalized with the groups last month, the Justice Department will notify prosecutors and convicted defendants or defense attorneys if an internal review panel or the two external groups find that FBI examiners “exceeded the limits of science” when they claimed to link crime scene hair to defendants in reports or testimony.

If so, the department will assist the class of prisoners in unprecedented ways, including waiving statutes of limitations and other federal rules that since 1996 have restricted post-conviction appeals. The FBI also will test DNA evidence if sought by a judge or prosecutor.

The review will prioritize capital cases, then cases in which defendants are imprisoned.

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 District of Columbia men 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 defense, greater transparency will “lessen skepticism” about the government’s motives. It also will be cheaper, faster and more effective because private parties can help track down decades-old cases.

Scientific errors “are not owned by one side,” he said. “This gives the same information to both sides, and they can litigate it.”

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 computerized in 1985.