In the coming weeks, the White House is expected to send Congress a final plan to "safely and responsibly" move dozens of detainees held in Guantanamo Bay to military and, possibly, federal prisons in the United States.

The move is part of President Barack Obama's pledge to shutter the 13-year-old detention facility, located on a remote part of the US Naval base in Cuba, before he leaves office. (The closing of Guantanamo would in some ways be symbolic; in 2011, Obama signed an executive order that made indefinite detention the law of the land.) Already, the plan has been met by fierce opposition by Republican lawmakers in Kansas and South Carolina, two states that are in the running to house detainees at military prisons located at Fort Leavenworth and the Naval Consolidated Brig in Charleston.

In an op-ed published in the Wall Street Journal last Sunday, US representatives Pat Roberts and Tim Scott, who represent Kansas and South Carolina respectively, said Guantanamo should remain open.

"Transferring these prisoners to the mainland puts the well-being of states in danger, posing security risks to the public and wasting taxpayer dollars," Roberts and Scott wrote.

It currently costs US taxpayers about $2.7 million annually to house a single detainee at Guantanamo.

Commander Gary Ross, a Department of Defense (DOD) spokesman, told VICE News that a decision on where in the US the military intends to house detainees has not been made.

"A small team of DOD officials will travel to relevant facilities to assess those facilities, identifying the changes required to make those facilities suitable to house law-of-war detainees, to estimate the costs in making the changes necessary, and to estimate costs to operate the facilities thereafter," Ross said.

There are 116 detainees still being held at Guantanamo, 52 of whom have been cleared for release or transfer. The remainder includes so-called "forever prisoners" — those too dangerous to release, yet too difficult to prosecute before military commissions — and high-value detainees, like self-professed 9/11 mastermind Khalid Sheikh Mohammed and his co-conspirators, who have been targeted for prosecution for war crimes.

Neither the Department of Defense nor the White House will publicly discuss the military and federal detention centers currently being considered, or the logistical and legal challenges that would be involved. But VICE News has found a long-forgotten report prepared by the nonpartisan Government Accountability Office (GAO), the investigative arm of Congress, that reveals the locations and addresses the challenges the US government will face in bringing detainees to the United States.

The report, titled "Facilities and Factors for Consideration If Detainees Were Brought to the United States," was released in November 2012 by Senator Dianne Feinstein, then the chairwoman of the Senate Intelligence Committee. She requested the study in 2008, months before Obama was sworn in as president and signed an executive order promising to close Guantanamo within a year.

"This report demonstrates that if the political will exists, we could finally close Guantanamo without imperiling our national security," Feinstein said in a prepared statement on November 28, 2012. "The GAO report makes clear that numerous prisons exist inside the United States — operated by both the Department of Defense and the Department of Justice — capable of holding… detainees who remain at Guantanamo in an environment that meets the security requirements."

The GAO report, which was based on site visits to Guantanamo and DOD prisons, along with interviews the office conducted with Defense and Justice Department officials, identified six military facilities where detainees could be held: Naval Consolidated Brig in Chesapeake, Virginia; Naval Consolidated Brig in Charleston; Naval Consolidated Brig in Miramar, California; Midwest Joint Regional Correctional Facility at Fort Leavenworth, Kansas; the Northwest Joint Regional Correctional Facility in Washington state; and the United States Disciplinary Barracks at Fort Leavenworth.

"These six facilities have the ability to secure inmates requiring additional controls such as protective custody or restricted communications," the GAO report said. "For example, the Midwest Joint Regional Facility at Ft. Leavenworth has eight specialized soundproof cells within its segregated housing unit that can be suited for individuals charged with high-profile crimes in order to limit their communications with other inmates."

(The military held Jose Padilla, a US citizen, as an enemy combatant in deplorable conditions at the Naval Consolidated Brig in Charleston for three years without charge after 9/11 over his alleged ties to terrorism.)

At Guantanamo, detainees are held in three "camps" that were modeled on US federal prisons: one where "compliant" detainees live in a communal setting; one where noncompliant detainees are isolated to single cells; and the top-secret Camp 7 that houses 15 high-value detainees who were formerly held in CIA custody. The GAO report said that with the exception of the classified Camp 7, DOD correctional facilities would be identical to the detention setting at Guantanamo if detainees were transferred. In a classified version of the GAO report, an additional section addressed Camp 7 and what would be entailed if high-value detainees like Mohammed were transferred to the US.

The report also makes clear that if Congress backs the plan and removes the legal restrictions currently in place that prohibit the Department of Defense from using its funds to transfer detainees to the US, it will be a difficult task to carry out:

First, US law prohibits the confinement of members of the armed forces in "immediate association" with foreign nationals. To comply with this law, DOD would need to relocate [Uniform Code of Military Justice (UCMJ)] inmates if detainees were to be moved to existing facilities. Additionally, according to DOD officials, for operational and policy reasons DOD prohibits the commingling of unprivileged enemy belligerents [which is how Guantanamo captives are identified] with those convicted of crimes…. DOD would also have to ensure that it maintained separation of detainees and UCMJ inmates while still conforming to international and US standards for access to medical care and recreation. According to DOD officials, provision of onsite medical care for detainees might present a challenge. All DOD corrections facilities provide medical services on site, although, they are limited in what they are equipped to provide. As a result, UCMJ inmates requiring more-complex medical treatments are transported to clinics or hospitals, but that might be challenging were Guantanamo Bay detainees to be treated in this way in the United States.

Guantanamo was chosen as the location to hold "war on terror" detainees after 9/11 because the US believed they would not be afforded any legal protections. On December 28, 2001, a young Justice Department attorney named John Yoo wrote a legal memo for the Pentagon's top attorney and explained that while the lease agreement gave the US "complete jurisdiction and control" of Guantanamo, it was still outside American sovereignty due to its location in Cuba. In other words, neither US nor Cuban courts would have jurisdiction over detainees held there if they tried to challenge the legality of their detention. The Supreme Court, in a landmark decision several years later, said Guantanamo detainees were entitled to challenge the legality of their detention through habeas corpus lawsuits.

Another issue the US would have to confront, the GAO report says, is ensuring compliance with international law. That means not only treating Guantanamo detainees humanely but also protecting them from "public curiosity." That may be difficult if the men are held at the two Fort Leavenworth prisons or at the Naval brigs in Miramar and Charleston, because those facilities are in "public view."

"For example, the general public might be able to view detainees utilizing the outdoor recreation areas, or detainee privacy and personnel anonymity may be compromised when transporting detainees off-site for medical care," the GAO report said.

If detainees are moved to DOD facilities in the US, the GAO report says, operations would have to be developed and implemented to ensure the safety of the general public. DOD officials told the GAO that one of their concerns is that the physical location of the detainees "could become a target for individuals and groups intent on harming the US military personnel involved in detainee operations — which could result in unintended harm to the general public."

"DOD's current ability to minimize risks to the public is attributable to Guantanamo Bay's remote location and limited access, whereas DOD's corrections facilities in the United States are generally located on active military installations in close proximity to the general public," the report says.

It also points out that if the DOD would want to conduct intelligence operations similar to those done at Guantanamo, the military prisons in the US do not have the means to accommodate the operations. DOD officials acknowledged that transferring detainees could increase "intelligence costs." The GAO report said intelligence operations at Guantanamo include the "voluntary interviews of detainees" to lend "support" to military commissions and parole board hearings for the "forever prisoners," whose cases are reviewed periodically.

At DOD's correctional facilities in the US, military personnel sentenced to prison are forced to provide services to the DOD, which has resulted in "almost $15 million in offset savings." But if detainees were moved to an existing DOD facility, in the US those operations would likely grind to a halt. DOD officials said if UCMJ prisoners are moved to another facility in order to make room for Guantanamo detainees, it might also require relocating work programs, "such as graphic arts, woodworking, as well as textile repair and embroidery for all Army uniforms and related gear."

DOD said it is also concerned that housing detainees on US soil could present risks to "administrative and training operations."

"For example, Ft. Leavenworth is home to the Joint Center for International Security Force Assistance, and it hosts international officials," the GAO report said. DOD officials have warned that the "objection of several foreign nationals to the Guantanamo Bay detention operations could affect these international exchanges."

According to the report, there are more than 98 federal correctional facilities, some of which currently hold more than 300 prisoners convicted of terrorism-related crimes, that could also house Guantanamo detainees. But the Justice Department does not "consider itself to have authority to maintain custody of DOD detainees under the" Authorization to Use Military Force (AUMF), the law passed by Congress that the US government cites to justify the detention of Guantanamo captives.

A Justice Department attorney in the Office of the Deputy Attorney General told the GAO that if detainees were to be transferred to facilities operated by the federal Bureau of Prisons (BOP), the US government would need to obtain additional legal authorities to hold the Guantanamo captives in federal prisons. It would be far cheaper than the estimated $2.7 million a year it currently costs to detain a detainee in Guantanamo. According to the GAO report, in 2011, on average it cost less than a $100 per day to hold a prisoner at a high-security federal correctional facility.

It's unclear whether any US federal prisons are being considered for holding detainees. The GAO report said the notorious Supermax prison in Florence, Colorado was one location where some Guantanamo detainees could be housed.

"New policies and practices that would be needed [if detainees were transferred to federal prisons] include standards governing (1) the holding of detainees who have not been charged with or convicted of violating U.S. law, (2) where and how each category of detainee would be housed, (3) religious and cultural accommodations, (4) visitation by the International Committee of the Red Cross, and (5) intelligence gathering and use," according to the GAO report.

VICE founder Shane Smith interviews US Secretary of Defense Ashton Carter

The US Marshals Service added that it would have to come up with a plan for transporting detainees to and from federal detention facilities, which could impact public safety and the safety of the captives. And if Guantanamo detainees were added to the federal detention population it could require BOP to "triple bunk" some of the existing inmates if the Guantanamo detainees needed to be confined to single cells.

However, the possible scenarios may be moot. The Justice Department said it was not even considering any BOP facilities due to transfer restrictions imposed by Congress.

Ross, the DOD spokesman, told VICE News the site visits by Pentagon personnel "are informational only and are being carried out so that we can establish a baseline assessment of costs and requirements that will help us finalize some key elements of our plan to close Guantanamo. The facilities also will be assessed for their ability to serve as military commission sites."

Wells Dixon, an attorney with the Center for Constitutional Rights, which has represented Guantanamo detainees for a decade, told VICE News that his organization has a "somewhat nuanced view" when it comes to transferring detainees to the US.

"We are absolutely opposed to bringing the cleared men into the US," he said. "They should be sent home or resettled into other countries. But we recognize that in order to close Gitmo, a small number of men need to be brought into the US for prosecution. We would support this as long as these men are afforded fair trials."

Dixon also said he recognizes that some of the lower-value Guantanamo detainees would rather be detained in the US because they would be afforded additional legal rights.

"The minute they set foot in the US, they will be entitled to full US Constitutional rights," Dixon said. "For them, it might be a greater opportunity to challenge the legality of their detention. Additional due process rights would impose an end point to their indefinite detention."

Stripping detainees of additional legal protections under the US Constitution has been something Congress has discussed if lawmakers were to lend support to the White House's proposal to transfer the captives to the US. But Dixon pointed out that one of his clients, Majid Khan, a high-value detainee, is already a legal US resident — the only one detained at Guantanamo — and limiting protections to which he would be entitled under the Constitution would be very difficult.

What is guaranteed is that the administration's attempts to shutter Guantanamo before Obama leaves office won't be easy. Beyond growing opposition among Republican lawmakers, governors Sam Brownback and Nikki Haley, of South Carolina and Kansas, vowed to fight the administration's plans to relocate Guantanamo prisoners to DOD facilities in their states. In a letter sent to Secretary of Defense Ashton Carter Tuesday, Brownback and Haley said Kansas and South Carolina are "sovereign states."

"We will not be part of any illegal and ill-advised action by this Administration, especially when that action relates to importing terrorists into our states," they wrote. "Please know that we will take any action within our power to make sure that no Guantanamo detainees are transferred to South Carolina or Kansas."

Follow Jason Leopold on Twitter: @JasonLeopold