The Pentagon is expected to release a plan next week on President Obama’s years-long effort to close the Guantanamo Bay detention center that suggests a Colorado prison dubbed “the Alcatraz of the Rockies” as one suitable site to relocate expected life-long detainees, Obama administration officials say.

Obama made a campaign promise in his 2008 White House bid to close the facility, arguing the move would be in the United States’ best financial, national security and foreign policy interests and in the name of justice -- considering some of the detainees have been held for nearly nine years without trial or sentencing.

However, critics of the promise, including many Republicans, fear transferring detainees to the U.S. mainland as part of an overall closure plan poses too much of a homeland security risk. They also say the president has yet to submit a closure plan and have been critical of the administration recently allowing some known terrorists to return to the Middle East.

The Florence, Colo., prison is among seven U.S. facilities in Colorado, Kansas and South Carolina being considered.

The Pentagon plan represents a last-gasp effort by the administration to convince staunch opponents in Congress that dangerous detainees who can't be transferred safely to other countries should be housed in a U.S.-based prison.

The United States opened the detention facility at Guantanamo Bay, Cuba, in the immediate aftermath of the 9/11 attacks to get suspected terrorists off the battlefield.

Congressional Republicans have been able to stop Obama from closing the facility by imposing financial and other restrictions.

White House Press Secretary Josh Earnest said this week that the administration is trying “very hard” to transfer 53 more detainees, among the 112 remaining, before the end of the year.

The rest are either facing trial by military commission or the government has determined that they are too dangerous to release but are not facing charges.

Any decision to select a U.S. facility would require congressional approval -- something U.S. lawmakers say is unlikely. However, Earnest also suggested that Obama has not ruled out the possibility of using an executive order to close the facility.

The Pentagon plan makes no recommendations on which of the seven sites is preferred and provides no rankings, according to administration officials.

A Pentagon assessment team reviewed the sites in recent months and detailed their advantages and disadvantages. They include locations, costs for renovations and construction, the ability to house troops and hold military commission hearings, and health care facilities.

Colorado's Centennial Correctional Facility has advantages that could outweigh its disadvantages, according to officials. But no details were available and no conclusions have been reached. The officials spoke on condition of anonymity because they weren't authorized to discuss the matter publicly.

The Florence, Colo., facility already holds convicted terrorists, including Unabomber Ted Kaczynski and Zacarias Moussaoui, one of the conspirators of the Sept. 11, 2001, attacks.

To approve a transfer, Defense Secretary Ash Carter must conclude that the detainees will not return to terrorism or the battlefield upon release and that there is a host country willing to take them and guarantee they will secure them.

Arizona Sen. John McCain is among the congressional Republicans who have asked for an administration plan for the shutdown of Guantanamo. And the Pentagon's assessment team visits over the last few months were part of the effort to provide options for the relocation of Guantanamo detainees.

"I've asked for six and a half years for this administration to come forward with a plan -- a plan that we could implement in order to close Guantanamo. They have never come forward with one and it would have to be approved by Congress," McCain said this week.

The facilities reviewed by the assessment team were the U.S. Disciplinary Barracks and Midwest Joint Regional Corrections Facility at Leavenworth, Kansas; the Consolidated Naval Brig, Charleston, South Carolina; the Federal Correctional Complex, which includes the medium, maximum and supermax facilities in Florence, Colorado; and the Colorado State Penitentiary II in Canon City, Colorado, also known as the Centennial Correctional Facility.

Colorado Republican Sen. Cory Gardner made clear this week that he opposes any move to relocate detainees to his state.

"I will not sit idly by while the president uses political promises to imperil the people of Colorado by moving enemy combatants from Cuba, Guantanamo Bay, to my state of Colorado," he said at a Capitol Hill news conference.

He also expressed concerns about the potential impact of such a move on the state’s judicial system and concerns about detainees potentially have to transported from the rural facility to downtown Denver to the federal courthouse for a hearing.

McCain and others have said that an executive order to shutter Guantanamo would face fierce opposition, including efforts to reverse the decision through funding mechanisms.

The prison at Guantanamo presents a particularly confrontational replay of that strategy. Obama would likely have to argue that the restrictions imposed by Congress are unconstitutional, though he has abided by them for years. The dispute could set off a late-term legal battle with Republicans in Congress over executive power, potentially in the height of a presidential campaign.

The Associated Press contributed to this report.