The FBI admitted Saturday it accidentally gave classified documents back to the American translator who pleaded guilty to taking them from the U.S. prison camp at Guantanamo Bay, Cuba.

Ahmed Fathy Mehalba, who was released from jail earlier this month, contacted the FBI's Boston office Tuesday after he realized agents had inadvertently given him the compact disc containing the secret files along with his personal property.

Mehalba had the disc in his possession for only a "matter of hours" before the FBI retrieved it, said Assistant U.S. Attorney Michael Ricciuti.

"Someone in the bureau obviously made a serious mistake," Ricciuti said.

Mehalba's lawyer, Michael Andrews, said the FBI's mistake was "very upsetting" to his client.

The FBI's Boston bureau disclosed its error in a press release Saturday.

FBI spokeswoman Gail Marcinkiewicz said the disc wasn't labeled "secret" — as all classified data should be — because prosecutors had to keep it in the same condition as when it was seized from Mehalba.

Authorities haven't said what type of information was stored on the disk, other than it concerns "national defense," said Ricciuti, who prosecuted Mehalba.

Mehalba, an Egyptian-born U.S. citizen who was working as a civilian Arabic translator at Guantanamo, was arrested at Boston's Logan International Airport on Sept. 29, 2003, after Customs agents found the disc in his possession.

The disc, one of 132 found in his luggage, contained hundreds of documents labeled "SECRET" or "SECRET/NOFORN," meaning no foreign government was allowed to see them. The disc itself wasn't labeled classified.

Last month, Mehalba pleaded guilty to one count of unauthorized possession of classified materials and two counts of lying to federal investigators and was sentenced to 20 months in prison. He was given credit for time already spent in jail and was released on March 10.

Mehalba told the judge he exercised "very poor judgment" in taking the disc from the base but meant no harm.

"There is no allegation he was involved in any kind of espionage," Ricciuti said.