The Pentagon broke the law when it transferred five Taliban detainees from Guantánamo Bay in exchange for prisoner of war Army Sgt. Bowe Bergdahl, according to a report from a government watchdog agency.
The law also prohibits the Pentagon from using appropriated funds to conduct any detainee transfer, unless the Defense secretary gives the 30 days of advance notice.
The Pentagon said it spent $988,400 on the detainee transfer, according to the report.
"As a consequence of using its appropriations in a manner specifically prohibited by law, DOD also violated the Antideficiency Act," the agency found. "DOD should report its Antideficiency Act violation as required by law."
That report, outlining the violation and any measures taken by the Pentagon in response, would then go to Congress and the president, according to a GAO spokeswoman.
The watchdog report is a legal opinion and will not lead to further measures, said the spokeswoman.
The seven-page finding is in response to a request by Republican lawmakers for an opinion, and is fueling fresh criticism of the Obama administration's decision to swap the five senior Taliban commanders for Bergdahl, who had been held by the Taliban since 2009.
Sen. Susan Collins (R-Maine), one of the nine Republican senators on the Defense Appropriations Subcommittee who requested the report, said it confirmed that Obama knowingly violated the law.
“It is extremely troubling that the president chose to ignore this notification requirement despite the previous bipartisan opposition to this ill-conceived swap," she said in a statement.
"It is highly likely that these men will return to the fight against our country after their year in Qatar. That is the assessment of the administration’s own intelligence experts," she said.
Collins said the argument that notifying Congress could have compromised the trade is "disingenuous" since dozens of administration officials knew about the swap in advance.
“The president’s decision is part of a disturbing pattern where he unilaterally decides that he does not have to comply with provisions of laws with which he disagrees,” she said.
The Obama administration has argued the swap was within the president's constitutional authority to protect U.S. personnel overseas.
Lawmakers from both parties were angered by the administration's move, and some argued the May release could jeopardize the lives of U.S. troops in Afghanistan. Others argued it was tantamount to negotiating with terrorists, though the administration negotiated the swap through Qatar.
Bergdahl is currently under investigation by the Army for possibly deserting his post in Afghanistan or being absent without leave, both of which are violations of the Uniform Code of Military Justice.
His investigation is expected to wrap up in September.
— This story was updated at 5:42 p.m.