The Obama administration reports that at least 12 terrorists from the military prison at Guantánamo Bay, Cuba — all released under George W. Bush — have killed an estimated half-dozen Americans, the Washington Post (WaPo) has learned from current and former U.S. officials on condition of anonymity.

Although the officials reportedly declined to provide a specific figure for former Guantánamo prisoners involved in attacks that killed and injured Americans, one is quoted as saying the number “was fewer than 15.”

The revelation follows the starling acknowledgement to the House Foreign Affairs Committee made by a top Pentagon official in March that detainees released from the U.S. military prison in Cuba have killed Americans.

Paul Lewis, the Pentagon’s special envoy for President Barack Obama’s relentless efforts to shut down the Guantánamo facility, declined to provide the GOP-led House panel with more details. The Obama administration has since refused to further elaborate publicly on the official’s comments, claiming the intelligence behind it is classified.

However, an unnamed administration official was quick to tell the Associated Press (AP) that Lewis was referring to an incident involving the release of an Afghan prisoner from Guantánamo under Obama’s predecessor.

Some U.S. lawmakers are urging the Obama administration to declassify the information about suspected attacks attributed to former Guantánamo prisoners that they have been privy to but are prohibited from discussing publicly.

“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 Guantánamo detainees,” wrote Sen. Kelly Ayotte (R-NH), a member of the Senate Armed Services Committee and a vocal opponent of Obama’s efforts to shut down the prison, in a letter to the president this week.

Some jihadists released from Guantánamo under President Obama have returned to terrorist activities, including one of the five high-level Taliban commanders released in the controversial exchange for deserter Sgt. Bowe Bergdahl.

Former and current U.S. officials, who spoke on condition of anonymity, provided the Post with “additional details about the suspected attacks [that Lewis was allegedly alluding to], 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.”

According to the Post, the officials also revealed that NATO troops who have been fighting alongside their American counterparts throughout most of the nearly 14-year-old Afghanistan war have also suffered casualties at the hands of the released prisoners.

One U.S. official familiar with the intelligence told the Post that, of the detainees suspected of attacking U.S. and allied personnel, nine are currently dead or in the custody of a foreign government.

The Pentagon official’s comments in March sparked criticism from Republicans opposed to closing the detention center.

“Some lawmakers see the violence against Americans as further evidence that the president’s plans for closing the prison are misguided and dangerous,” points out WaPo. “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 political opposition to its plans.”

President Obama continues to push ahead in his efforts to shut down the Guantánamo detention center, commonly known as Gitmo.

The Guardian reported in late May that his administration is secretly preparing to transfer up to 24 prisoners out of Gitmo by the end of the summer, a move that would bring the detainee population down to 56 from the 242 who were held there when Obama took office.

Earlier this year, the Pentagon submitted to Congress the Obama administration’s proposal for closing the Guantánamo detention center, which would require transferring prisoners to the United States, a move that is opposed by many lawmakers and is prohibited by law.

Currently, 80 terrorists are being housed at the prison.

In March, the office of the Director of National Intelligence (ODNI) reported that as of mid-January, about 3 out of every 10 detainees were suspected or confirmed to have re-engaged in terrorist activities after being transferred out of Gitmo.

Although that includes some terrorists released under President Obama, his administration suggests that none have been linked to attacks against the United States or its allies.

“Human rights activists say the statistics are suspect and cannot be verified because the administration provides almost no information about whom it is counting and why,” notes the Post.