In this file image, taken from video obtained from the Voice of Jihad Web site, Sgt. Bowe Bergdahl sits in a vehicle in eastern Afghanistan before being swapped for Taliban prisoners in 2014. (Associated Press)

House lawmakers are threatening to slash Defense Department funding by about $500 million next year if Pentagon officials don’t hand over documents related to a probe into the controversial prisoner swap that freed five Taliban detainees in exchange for captive U.S. soldier Bowe Bergdahl.

The Republican leadership of the House Armed Services Committee has tucked a provision into an early version of an annual defense bill that would cut funding for the Office of the Secretary of Defense by 25 percent in fiscal 2016, committee staff members said, unless the Pentagon provides unredacted e-mails related to the 2014 swap and additional information into the legal reasoning behind the exchange.

The unusual move illustrates the depth of Republican anger over the decision to transfer the five detainees from the military prison at Guantanamo Bay, Cuba, to the Gulf state of Qatar. Critics allege the Obama administration endangered U.S. security and flouted U.S. law in failing to provide Congress with a mandated notification 30 days prior to the transfer.

While the release of Bergdahl, who was held in Pakistan under difficult conditions for almost five years, received initial praise, the deal quickly came under attack from lawmakers angry about being left out of the loop and from those skeptical about the circumstances of Bergdahl’s initial disappearance. Bergdahl has since been charged with desertion for walking off of his base in a remote area of Afghanistan in 2009.

Only days after the swap took place, the committee launched an investigation into whether the administration had broken the law.

In a June 9, 2014, letter to then-Defense Secretary Chuck Hagel, then-committee Chairman Howard “Buck” McKeon (R-Calif.) ordered the Pentagon to turn over e-mails and internal documents related to the transfer.

Lawmakers were particularly interested in internal deliberations surrounding the decision to forego congressional notification. Although a probe into the matter by the independent Government Accountability Office found that the administration had violated federal rules, the administration has maintained that the transfer was legal and necessary to protect Bergdahl’s safety.The Obama administration was also concerned that plans for the swap could become public, something they feared would cause the Taliban to scuttle the deal. On May 31, militants handed over Bergdahl — clothed in local garb, his head shaved — to Special Forces soldiers on a field in eastern Afghanistan.

Hagel, speaking at a tense hearing last June, told lawmakers that officials had accelerated their efforts to liberate the country’s only prisoner of war after they received a video in January 2014 showing that Bergdahl’s mental and physical health had worsened.

After years of on-and-off contact with Taliban representatives or their intermediaries, officials said, the transfer came together quickly in the final weeks of May 2014.

“After the exchange was set in motion, only 96 hours passed before Sergeant Bergdahl was in our hands,” Hagel said. “We believed this exchange was our last, best opportunity to free him.” Bergdahl is now awaiting a preliminary Article 32 hearing, the military equivalent of a grand jury proceeding.

Hagel also assured lawmakers that the Pentagon had obtained legal guidance from the Justice Department indicating that the administration could go ahead with the transfer without notifying Congress ahead of time, based in part on President Obama’s authority to act to protect the lives of Americans overseas.

For much of the past year, committee staff members have sifted through e-mails and documents that led up to that moment. They have also called in officials for multiple briefings and visited Qatar to investigate.

But staff members said their investigation had been impeded by heavy redactions to more than 3,000 pages of classified and unclassified e-mails provided to the committee. They said the redactions obscured information about legal discussions, negotiations with the Qatari government and preparations for briefing Congress — the very information they say they require for their probe.

In a sample of e-mails viewed by The Washington Post, substantial portions of messages had been redacted, including the names of recipients and senders as well as portions of text contained in the bodies. The Post reviewed only a small number of the unclassified e-mails that the Pentagon provided to the committee.

One e-mail relates to arrangements for a news briefing about the prisoner swap. An official writes, “We should not use this line, which is just a pointless stick in Congress’ eye.” What follows is redacted.

Obtained by The Washington Post

In response, Rep. Mac Thornberry (R-Tex.), the new committee chairman, drew up the punitive language in the initial version of the fiscal 2016 National Defense Authorization Act. Debate on the bill will begin next week.

Lt. Col. Joe Sowers, a Pentagon spokesman, said the department had provided over 3,600 pages of documents, had sent officials to take part in multiple hearings and briefings about the swap, and had provided the committee with its legal analysis both in person and in writing. He said document redactions had been “minimal.”

“The department has committed to working with the [committee] to accommodate their requests for information,” he said.

Cutting funding for the Office of the Secretary of Defense would affect multiple offices that report to Defense Secretary Ashton B. Carter, including those responsible for global security policy, defense budgeting, weapons procurement and military personnel issues. Funding for the office, which employes about 2,100 people, was $1.9 billion in fiscal 2015, a Pentagon official said.

A committee staff member, speaking on the condition of anonymity to discuss legislation that has not been made public, said that generally positive relations between lawmakers and the Pentagon didn’t mean “we’re not going to take serious steps.”

Some Republican lawmakers have also expressed concern about the activities of the former detainees since they were freed in Qatar.

Rep. Adam Smith (D-Wash.), the committee’s top Democrat, does not support Thornberry’s move to withhold hundreds of millions of dollars in defense funding.

Michael Amato, a spokesman for Smith, said administration officials had taken part in 29 meetings or briefings.

“The Administration has been forthcoming throughout the course of the majority’s extensive investigation,” Amato said. “Given our ongoing military engagements around the globe, punishing the Secretary of Defense for something outside of his control is shortsighted and unwise.”

Even if few Democrats support Thornberry’s language, the provision is expected to remain in the bill as it makes its way through the Republican-controlled House.