President Obama’s administration believes that at least 12 detainees released from the prison at Guantanamo Bay, Cuba, have launched attacks against U.S. or allied forces in Afghanistan, killing about a half-dozen Americans, according to current and former U.S. officials.

In March, a senior Pentagon official made a startling admission when he acknowledged that former inmates at Guantanamo, also known as Gitmo, were responsible for the deaths of Americans overseas.

The official, Paul Lewis, who oversees Guantanamo issues at the Defense Department, provided no details, and the Obama administration has since declined to elaborate publicly on his statement because the intelligence behind it is classified.

But The Washington Post has learned additional details, including the approximate number of detainees and victims involved and the fact that while most of the incidents were directed at military personnel, the dead also included one American civilian: a female aid worker who died in Afghanistan in 2008. The officials, who spoke on the condition of anonymity, declined to give an exact number for Americans killed or wounded in the attacks, saying the figure is classified.

Lewis’ statement had drawn scrutiny from some lawmakers who see the violence against Americans as further evidence that the president’s plans for closing the prison are misguided and dangerous. They also describe the administration’s unwillingness to release information about the attacks as another instance of its use of high levels of classification to avoid discussion of a politically charged issue that could heighten opposition to its plans.

One U.S. official familiar with the intelligence said that nine of the detainees suspected in the attacks are now dead or in foreign government custody. The official would not specify the exact number of detainees involved but said it was fewer than 15. All of them were released from Guantanamo Bay under the administration of George W. Bush.

The official added: “Because many of these incidents were large-scale firefights in a war zone, we cannot always distinguish whether Americans were killed by the former detainees or by others in the same fight.”

Military and intelligence officials, responding to lawmakers’ requests for greater details, have provided lawmakers with classified documents about the suspected attacks. One memo from the National Counterterrorism Center, which was sent to the House Foreign Affairs Committee after Lewis’ testimony, described the attacks, named the detainees involved and provided information about the victims without giving their names.

But lawmakers are prohibited from discussing the contents of that memo because of its high classification level. A similar document provided last month to the office of Sen. Kelly Ayotte, R-New Hampshire, an opponent of Obama’s Guantanamo policy, was so highly classified that even her staffers with top-secret clearance were unable to read it.

“There appears to be a consistent and concerted effort by the Administration to prevent Americans from knowing the truth regarding the terrorist activities and affiliations of past and present Guantanamo detainees,” Ayotte wrote in a letter to Obama this week,.

Just under 700 detainees have been released from Guantanamo since the prison opened in 2002; 80 inmates remain.

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