The arrest and abrupt release of an American lawyer who was initially linked by fingerprints to the deadly train bombing in Madrid may reflect the limitations of computerized fingerprint matching, experts say.
Brandon Mayfield was jailed two weeks ago after a computer identified his prints on a bag of detonators in Spain. But a U.S. judge let him go Thursday after Spanish officials said fingerprints belonging to an Algerian were on the bag.
Fingerprint experts said that if Mayfield was the victim of a fingerprint identification mistake, such errors are likely to become more common with the expanding use of computers that compile police records from around the world into huge, searchable databases.
"We're all baffled," said Pat Wertheim, a fingerprint examiner for the Arizona Department of Public Safety when asked about the partial match that put Mayfield in jail. "The fingerprint community is really anxious to see these prints and try to understand what has happened."
In theory, no two fingerprints are the same, but some can look similar, according to the Web site of the Scientific Working Group on Friction Ridge Analysis, a group for fingerprint examiners.
No statistics exist on false fingerprint matches in the United States, but mistakes are believed to be rare, in large part because all fingerprints are checked by human examiners who make the final decision on a match.
But the federal database that tied Mayfield to the plastic bag in Madrid holds tens of millions of fingerprints. The computer compares curve angles and patterns to produce a list of possible suspects.
"Obviously, the larger the database, the greater the possibility of two fingers having roughly similar sets of coordinates," Wertheim said. "It's an issue that has troubled some of us in the business."
Once a computer identifies possible matches, the science of matching is much the same as it was when fingerprints were first used in a U.S. courtroom in Chicago in 1911: It is up to humans to review two blobs of squiggly lines and decide if they are the same.
Mayfield, 37, was released pending grand jury proceedings and remains a material witness, according to federal officials. He was initially arrested on a material witness warrant that forbids prosecutors and defense attorneys from publicly discussing his case.
Spanish officials have not said whether the Algerian man's fingerprints were the only prints on the bag, and the FBI has not said if it had additional evidence against Mayfield besides the partial print.
The bag contained detonators similar to those used in the March 11 blasts, which killed 191 people and injured 2,000 others.
FBI agents probably saw the match as plausible because Mayfield is a Muslim, and the bombings were linked to Islamic terrorists, and because Mayfield had represented a terrorism suspect in Portland two years ago, Wertheim said.
Spanish authorities first cast doubt on the match, saying they found only eight points of similarity between Mayfield's print and the one on the bag. The FBI said it found 15 coinciding points.
Wertheim said that discussion was perplexing, because fingerprint analysts have largely abandoned the 16-point method of comparing prints after a 1989 British study cast doubt on its reliability.