In May 2004, the Federal Bureau of Investigation (FBI) arrested Brandon Mayfield, a lawyer, as a material witness in an investigation of terrorist attacks on commuter trains in Madrid, Spain. The FBI Laboratory maintained that Mayfield’s fingerprint was found on a bag of detonators in Madrid that was connected to the attacks. Two weeks after Mayfield was arrested, the Spanish National Police advised the FBI that it had identified another individual named Ouhnane Daoud as the source of the fingerprint. After the FBI examined Daoud’s fingerprints, it realized that it had made a mistake and released Mayfield from custody.

Following this misidentification of Mayfield, the Office of the Inspector General (OIG) investigated the causes of the misidentification and issued its written conclusions. The OIG identified the following six primary causes of error:

1. Although Mayfield and Daoud did not have identical fingerprints, they did, nevertheless, have very similar-looking prints;

2. After the FBI found as many as 10 points of unusual similarity between Mayfield’s fingerprint and the fingerprint located on the bag of detonators, "the FBI examiners began to ‘find‘ additional features in [the fingerprint on the bag] that were not really there, but rather were suggested to the examiners by features in the Mayfield prints. As a result of this process, murky or ambiguous details in [the fingerprint on the bag] were erroneously identified as points of similarity with Mayfield’s prints."

3. The FBI fingerprint examiners "apparently misinterpreted distortions in [the fingerprint on the bag] as real features corresponding to [extremely tiny details] seen in Mayfield’s known fingerprints." Thus, whereas error #1 had to do with comparatively large fingerprint details, error #3 had to do with extremely tiny details.

4. FBI fingerprint examiners are taught to adhere to the "one discrepancy rule" according to which "a single difference in appearance between [an unknown] print and a known fingerprint must preclude an identification unless the examiner has a valid explanation for the difference." In Mayfield’s case, the examiners failed to adhere to this rule when they accepted an "extraordinary set of coincidences" and "cumulatively required too many rationalizations to support an identification with the requisite certainty."

5. As noted in error #2 above, the FBI found as many as 10 points of unusual similarity between Mayfield’s fingerprint and the fingerprint located on the bag of detonators. "However, the limited clarity of [the fingerprint on the bag] prevented the examiners from making an accurate determination of the type of many of these points (that is, whether they were ending ridges or bifurcations)."

6. Although the Spanish National Police advised the FBI on April 13, 2004 that the fingerprint on the bag of detonators did not match Mayfield’s prints, the FBI nevertheless arrested Mayfield more than three weeks later on May 6, 2004. In what is certainly an understatement, the OIG concluded that "the FBI Laboratory’s overconfidence in the skill and superiority of its examiners prevented it from taking the [April 13 report] as seriously as it should have." According to the OIG, what the FBI should have done was:

Determine precisely why the Spanish National Police examiners believed that Mayfield’s fingerprints did not match the print on the bag before arresting him; and

Have a new FBI examiner examine the fingerprint on the bag in order to verify whether or not it was Mayfield’s.

In reviewing the OIG’s report, the two things that stand out to me the most are:

The fact that the six errors discussed in this article were committed by not just one person but by four people including: a fingerprint examiner with the FBI Latent Print Unit, a second FBI Latent Print Unit examiner, a Unit Chief in the FBI Latent Print Unit, and an independent expert appointed by the judge to review the FBI’s fingerprint identification.