The FBI will examine hair sample evidence from more than 2,000 cases dating as far back as 1985. McClatchy reports that under the initiative the bureau will investigate cases in which hair samples helped secure convictions, including some that led to the death penalty. The US Department of Justice will waive its normal deadlines for appeal in order to give "wrongly convicted people a fair chance of review."

Free DNA testing will be offered when evidence is in doubt

Thousands of cases between 1985 and 2000 relied on hair samples, and the study will focus on "whether analysts exaggerated the significance" of the samples, or in some cases reported the results inaccurately. Free DNA testing will be offered in any cases where the FBI is found to be in error. Peter Neufeld, a co-director at the non-profit Innocence Project, tells McClatchy that "the government's willingness to admit error and accept its duty to correct those errors in an extraordinarily large number of cases is truly unprecedented."

The FBI could have overstated the significance of hair sample evidence

FBI Special Agent Ann Todd says "there is no reason to believe the FBI Laboratory employed 'flawed' forensic techniques," adding that microscopic hair analysis is "a valid forensic technique and one that is still conducted at the lab" alongside DNA testing. Todd notes "the purpose of the review is to determine if FBI Laboratory examiner testimony and reports properly reflect the bounds of the underlying science." Defense attorneys claim that laboratory analysts "overstated the significance" of the evidence.

It's not clear how many cases could be reversed, but the Innocence Project says that almost a quarter of individuals exonerated by DNA evidence were originally convicted in part thanks to microscopic hair analysis. Some had died in custody.

Read next: "Could you patent the sun?" Inside the Supreme Court case on patenting DNA