Tom Vanden Brook

USA TODAY

Twelve former detainees released from the military prison at Guantanamo Bay, Cuba, have attacked and killed Americans serving in Afghanistan, a senior U.S. official told USA TODAY Thursday.

The official, who spoke on condition of anonymity because officials were not authorized to speak publicly, said the 12 former detainees were released before 2008, and the attacks occurred before 2009.

In a report published online Wednesday, the Washington Post said a senior Defense official told Congress about the attacks in March.

The Pentagon would not release detailed information about the detainees or the attacks because it is classified, said Army Lt. Col. Valerie Henderson, a Pentagon spokeswoman.

"While we won't be able to discuss classified information, we take any incidence of reengagement very seriously," Henderson said in a statement. "We have a thorough, interagency process designed to assess risk prior to any transfer. And we work in close coordination through military, intelligence, law enforcement, and diplomatic channels to reduce the risk of reengagement and to take follow-on action when necessary."

Fewer than 15 of almost 700 former detainees at Guantanamo have been found to have attacked U.S. or allied forces, said a second U.S. official familiar with the issue but not authorized to speak publicly. All of the attacks occurred in Afghanistan. It’s impossible to tell if the former detainees were responsible for killing Americans because their deaths occurred in large battles. Nine of the detainees implicated in the attacks are dead or are being held by foreign governments.

President Obama has tried to have the facility closed taking office but has been blocked by Congress. He has, however, released as many detainees as possible who have gone through the vetting procedures for release. There are now 80 detainees at Guantanamo.

U.S. releases two Libyans from Guantanamo Bay, bringing population to 89