In a written statement the agency said: "The FBI apologizes to Mr. Mayfield and his family for the hardship that this matter has caused."

Robert Jordan, the FBI agent in charge in Oregon, read from the statement Monday at a news conference in Portland, but substituted the word "regretted" for apologizes.

Asked if the distinction was merely semantic, he said, "Yes, it is semantic."

He then answered "yes" when asked if the FBI was apologizing to the Mayfield family, and said the agency would "reach out to (the family) and seek an opportunity," to apologize directly.

Jordan spoke just hours after a judge dismissed the case against Mayfield, the 37-year-old lawyer from the Portland suburb of Aloha who had been held as a "material witness" since May 6 in the Madrid bombing cases, which killed 191 people and injured about 2,000 others.

Mayfield, a former Army lieutenant, was released from custody last week. But he was not altogether cleared of suspicion; the government said he remained a material witness and put restrictions on his movements.

Those restrictions were lifted Monday.

Mayfield's attorney, Public Defender Steven Wax, said the FBI had told him the error was the first ever by the agency's fingerprint analysis group.

Jordan said that the FBI's initial determination that Brandon Mayfield's fingerprint was on a bag of detonators found near the Madrid train station was "based on an image of substandard quality."

Court documents released Monday suggested the mistake first sprang from an error by the FBI's supercomputer for matching fingerprints, the Automated Fingerprint Identification System.

According to the affidavit for a search warrant, FBI fingerprint examiners at the agency's lab in Quantico, Va., had used the system to search for possible matches to an unknown digital image of a fingerprint furnished by the Spanish authorities on March 17, six days after the bombings.

The system returned 15 possible matches, including prints belonging to Mayfield, on file from a 1984 burglary arrest in Wichita, Kan., when Mayfield was a teenager.

Three separate FBI examiners — Terry Green, Michael Wieners and John T. Massey — narrowed the identification to Mayfield, according to Jordan.

The FBI maintained their certainty even as Spanish authorities said by mid-April that the original image of the fingerprint taken directly from the bag using dust and glue did not match Mayfield's, Wax said.

The Spanish had expressed these doubts roughly three weeks before the FBI arrested Mayfield.

In court papers, federal agents expressed no doubts about the digital copy of fingerprints they possessed.

"I have been advised that the FBI lab stands by their conclusion of a 100 percent positive identification," FBI Special Agent Richard K. Werder said in the affidavit filed in U.S. District Court in Portland.

Kenneth Moses, a court-appointed fingerprint expert, also confirmed that a digital image of the print from Madrid matched those on file for Mayfield's.

But last week, Spanish authorities said they matched the fingerprints of an Algerian man to those on the bag.

Jordan said FBI examiners flew to Spain, viewed the original print pattern of the fingerprint on paper, and agreed that it was not Mayfield's.

U.S. Attorney in Oregon Karin Immergut said the court was promptly notified and Mayfield freed. Immergut also denied Mayfield had been targeted because he is Muslim.

As additional evidence in support of Mayfield's arrest, the FBI had pointed to Mayfield's attendance at a local mosque, his advertising legal services in a publication owned by a man suspected to have links to terrorism and a telephone call his wife placed to the Ashland branch of Al-Haramain, a Saudi Arabia-based Islamic charity with suspected terrorist ties.

The affidavit states there is no record of any foreign travel by Mayfield but concludes: "Since no record of travel or travel documents have been found in the name of Brandon Bieri Mayfield, it is believed that Mayfield may have traveled under a false or fictitious name."

But the affidavit lists no evidence or testimony indicating any false documents or any travel.

Jordan said Monday the FBI has entirely dropped its investigation, apparently acknowledging the agency has discounted these other suspicions.

Wax called the FBI's list of secondary evidence nothing but baseless "innuendo."

"We need to know more about how this happened. All of us in this country need to know more about how this type of mistake can be made," Wax said.

"The climate of fear of terror makes this a cautionary tale about the way in which that fear can ensnare an innocent person in the type of abuse to which Mr. Mayfield was subjected," he said.

Concerns surfaced early on, Wax said, that the print found on the bag of detonators had been "overlaid" with another print, obscuring details and making a match difficult — and far less certain then the FBI represented.

He said his legal team was preparing to challenge the FBI's lab before the agency conceded its own mistake.

Also of concern, he said, was the possibility that his client could have been singled for investigation and subsequent arrest because he is Muslim. "It's a major civil rights issue," he said.

Wax said Mayfield believes he was not only arrested, but also subjected to so-called "sneak and peak" searches where agents enter a home with no obligation to immediately tell the owner. They are allowed under the USA Patriot Act. Mayfield may sue the government, Wax said.