(CNN) -- The Pentagon broke federal law by exchanging five Taliban detainees for Army Sgt. Bowe Bergdahl without giving Congress the appropriate notice, Congress' independent investigative arm announced Thursday.
Susan Poling, general counsel for the U.S. Government Accountability Office, wrote in a letter to nine Republican senators that the Pentagon should have notified "the relevant congressional committees at least 30 days in advance of the transfer."
Moreover, Poling noted that the GAO also concluded the Defense Department broke the law by using "appropriated funds to carry out the transfer when no money was available for that purpose."
Poling noted that her office wasn't weighing in on Defense Secretary Chuck Hagel's decision to carry out the prisoner swap, but rather his "responsibility to comply with a notification condition on the availability of appropriations to transfer individuals from Guantanamo Bay."
Talking to CNN on Thursday evening, Pentagon spokesman Rear Adm. John Kirby noted Hagel's previous admission "that we probably could have done a better job keeping Congress informed. Absolutely."
But that doesn't mean the Defense Department did anything illegal, Kirby said. He stated officials felt they had "to move quickly" out of concern for Bergdahl's life. The Pentagon said that President Barack Obama's administration felt "it was necessary and appropriate to forgo 30 days' notice" to do so.
"We ... believe that it was lawfully done, lawfully conducted," Kirby said of the exchange. "And this was a judgment that was shared by the Justice Department."
Poling's letter was in response to a June 13 request from the GOP senators for her opinion on the matter.
Captured in Afghanistan early in the summer of 2009, Bergdahl was freed almost five years later in exchange for five men who were being held at the U.S. military prison in Guantanamo Bay, Cuba.
At the time of his release, the 28-year-old was the longest-held U.S. soldier since the Vietnam War.
The swap stirred sharp criticism, especially among Republicans who questioned whether the U.S. government might encourage its enemies to take more Americans captive. Others -- including members of his unit -- accused him of deserting his comrades in the midst of a war.
An Army fact-finding investigation conducted in the months after his disappearance concluded he left his outpost deliberately and of his own free will, according to an official who was briefed on the report.
There has been no definitive conclusion Bergdahl was a deserter. He met for two days earlier this month with an Army investigating officer who has been reviewing his case.
Unless he requests an extension, the investigating officer has until mid-August to complete the probe. A report would typically be issued about a month later.
As for Bergdahl, he's back in the active military. Following his release in late May, the Idaho native underwent medical care and mental counseling at an Army hospital in San Antonio before taking a desk job at Fort Sam Houston, according to military officials.
Sen. Saxby Chambliss made no mention of Bergdahl -- including how he went missing, his time in captivity or his future -- in a statement Thursday on the GAO's findings.
The Georgia Republican did, however, reference "the Taliban Five" and repeated Poling's assertions about the Defense Department handling of this matter.
In doing so, he reiterated a claim made by many in his party -- as evidenced by the GOP-led House giving the go-ahead earlier this summer to sue Obama over executive orders -- that the administration has often overreached its authority.
"While the President has a habit of ignoring laws relating to domestic policy, such as health care and immigration, this latest overreach regarding our national security has dangerous implications," Chambliss said. "The United States has a long-standing policy of not negotiating with terrorists for good reason, and these senior Taliban leaders will soon rejoin the fight."