FEDERAL · FBI · FINGERPRINT MISIDENTIFICATION

The FBI called a fingerprint match '100%'—and arrested the wrong man.

AFIS candidate → examiner confirmation → investigation and detention → Spanish police identify the true source

After the March 2004 Madrid train bombings, the FBI entered a latent fingerprint from a bag associated with the attack into its Automated Fingerprint Identification System. Brandon Mayfield, an attorney in Portland, Oregon, emerged as a candidate. FBI examiners then concluded that Mayfield's print matched the latent print and treated the identification as certain.

The FBI opened an investigation, obtained warrants and arrested Mayfield as a material witness. He was detained for approximately two weeks. Spanish National Police had already reached a negative conclusion on the Mayfield comparison; they later identified Algerian national Ouhnane Daoud as the actual source of the latent print, and the FBI withdrew its identification.

The Justice Department Inspector General concluded that unusual similarity between the prints contributed to the error, but also found examiner mistakes, circular reasoning, excessive reliance on tiny details, overlooked discrepancies and overconfidence that prevented the FBI from adequately reconsidering the match after Spanish police disagreed.

Why it matters: A biometric system can produce a candidate; human experts can then turn that candidate into institutional certainty. The danger is not only an algorithmic false positive—it is what happens when people begin reasoning from the assumption that the biometric must be right.

Primary sources: DOJ OIG — Brandon Mayfield review · DOJ OIG summary

WHY THIS LIBRARY EXISTS

This page preserves the anecdote and its caveats separately from the shorter Why Privacy Matters explainer. The goal is to keep the argument readable without hiding the receipts.