On March 11, 2004, bombs tore through four commuter trains in Madrid, killing nearly 200 people and wounding more than 1,400. Investigators recovered a fingerprint, designated Latent Fingerprint 17, from a bag of detonators in a stolen van, and forwarded a digital image through Interpol to the FBI's crime laboratory in Quantico, Virginia. Searching it against a database of more than 47 million people's fingerprints, an examiner found a candidate: fourth on a computer-generated list of twenty. On March 19, the FBI declared a match. The print belonged to Brandon Mayfield, an Oregon attorney with no known connection to Spain.
FBI policy required a second examiner to verify any identification before it left the laboratory. The man assigned, a retired examiner working under contract, already knew a colleague had called the print a match before he examined it himself; he agreed. A unit chief then reviewed both examiners' conclusions and signed off as well, without running his own full comparison. FBI examiners later told the Justice Department's Inspector General that declining to confirm a colleague's identification almost never happened. Three examiners looked at the print. Only the first one didn't already know what answer he was supposed to find.
Spain's own forensic laboratory was examining the same print, and on April 13 it reported the opposite: negativo. The FBI lab reviewed its conclusion, decided it still held, and told its counterterrorism division it remained completely confident in the match, without first learning the basis of Spain's doubts. A unit chief flew to Madrid six days later to walk Spanish examiners through the reasoning, not to reopen the comparison. By April 19, agents had confirmed Mayfield hadn't traveled abroad in years, both passports long expired, and revised their theory instead: perhaps the bag had crossed the Atlantic without him.
The FBI arrested Mayfield as a material witness on May 6, with Spain still unpersuaded. On May 19, the same day a court-appointed American fingerprint expert testified under oath that the match was sound, Spanish police delivered different news: the print belonged to Ouhnane Daoud, an Algerian whose prints had surfaced weeks earlier in an unrelated raid. The FBI's own overnight reexamination confirmed it. Mayfield was released from custody the next day; four days after that, the government formally withdrew the identification, dismissed the case, and the FBI publicly apologized. He had spent two weeks in custody for a fingerprint that was never his.
The Justice Department's Inspector General later found that the print's genuine, unusual resemblance to Mayfield's prints was only part of the story. Investigators found no evidence that the FBI Laboratory seriously explored the possibility it had erred after Spain's April 13 warning; a better response, the report concluded, would have started by learning the full basis of Spain's disagreement before reaffirming the identification, and would have handed the print to an examiner who hadn't already committed to an answer. The FBI did neither. A laboratory that has already decided the answer isn't checking anything. It's just waiting to be told it was right.