A review of the FBI's use of hair analysis in criminal cases has discovered a persistent bias amongst members of an elite FBI forensic unit, of whom 26 out of 28 have been found to have overstated forensic matches in such a way that favoured the prosecution over the defence.

In upwards of 95% of 268 trials which have so far been reviewed, this was shown to have been the case, and of those, 32 cases resulted in a death penalty. Around 14 of the defendants in those cases have now died in prison or been executed – although this error alone does not mean that they were innocent, it casts serious doubts on the verdicts of hundreds of cases.

Since 1989, 329 people have been exonerated on the basis of DNA testing, and in many of those cases, hair analysis and techniques similar to that were found to have been at fault. Both hair and bite mark comparisons are based on the analysis of patterns, and as such are quite subjective and open to interpretation – a flaw which appears to have lead to the wrongful conviction and possible execution of a number of innocent men and women in trials which the FBI unit was involved in.

The organisation has admitted that the hair examiners did not have a written standard which defined the correct way to explain results in a court of law, leading to the somewhat biased testimony which was given. As of last year, the FBI was intending to hold all forensic disciplines to the same standard, and required written guidelines to be in place both for lab reports and testimony regarding the results of forensic testing.

A former prosecutor, Richard Blumenthal, has suggested that the justice department in the USA and the FBI ought to notify every defendant in the 2,500 cases in question that the problem had come to light, even if a review of their specific case had not yet been completed.

He said, “These findings are appalling and chilling in their indictment of our criminal justice system, not only for potentially innocent defendants who have been wrongly imprisoned and even executed, but for prosecutors who have relied on fabricated and false evidence despite their intentions to faithfully enforce the law.”