Back in June, Republican members of the Senate Appropriations Committee sent a letter questioning the move and asking the GAO to identify possible violations of the federal Antideficiency Act.
The 7-page review, released by the GAO Thursday, found the DOD violated the requirement to notify Congress of the transfer at least 30 days in advance when it conducted the transfer of the five Taliban prisoners from Guantanamo Bay to Qatar at the cost of $988,400.
A spokesman for the GAO said the report was a legal finding, and it would not take further action.
At the time, the White House said they chose not to divulge the specifics of the deal as it came together, citing threats from the Taliban if any news should leak. Secretary of Defense Chuck Hagel said Bergdahl's deteriorating health and fears that the window to bring him home would close played into the Pentagon's decision to move as quickly as it did.
"It was our judgment that if we could find an opening and move very quickly with that opening, that we needed to get him out of there essentially to save his life," he said at the time. "I know President Obama feels very strongly about that, I do as well."
The Obama administration's celebratory mood around the news of Bergdahl's return in early June quickly soured as lawmakers from both parties piled on with accusations that the swap had violated the law.
Members of Congress criticized the terms of the deal, worrying the five detainees would return to their activities with the Taliban and the swap would encourage other extremists to take U.S. military personnel hostage.
"You've sent a message to every Al Qaeda group in the world that there is some value now in that hostage," House Intelligence Committee Chairman Mike Rogers, R-Mich., said.
House Armed Services Chairman Howard Buck McKeon, R-Calif., promised hearings on the issue.
"My perception is he broke the law by not informing Congress 30 days before," he said. "We passed [the NDAA] last year. It passed overwhelmingly on the floor and through the Senate and the president signed it. And although he said now he had a disclaimer along with it that he apparently didn't support the law, he did sign it."
National Security Advisor Susan Rice said the administration had examined the legality of the the move before making the swap, adding that Congress had been told negotiators might move swiftly if the right opportunity arose.
"It was determined that it was necessary and appropriate not to adhere to the 30-day notification requirement because it would have potentially meant that the opportunity to get Sgt. Bergdahl would have been lost," Rice said. "In fact, we had briefed Congress in the past about this potential."
But lawmakers weren't swayed by the administration's argument, and fired off a letter asking for the GAO review.
"In addition to the national security implications associated with President Obama's release of these Taliban fighters, the legal and constitutional issues need to be addressed," said Sen. Thad Cochran, R-Miss., the vice chairman of the Senate Defense Appropriations Subcommittee, in June. "We simply can't have the President ignoring duly enacted laws that he himself has signed."
Read the report: