The clock is ticking toward the Feb. 23 deadline for the administration to provide Congress with details of how it will close the prison at Guantanamo Bay, Cuba. But it's still unclear where detainees who can't be released will be sent.

Rep. Mac Thornberry, R-Texas, sent a letter this week to President Obama reminding him that lawmakers expect a plan on their desks in just over two weeks, as mandated by the defense policy act. By Feb. 23, Congress should have specific details and cost estimates to close the military prison.

"Press reports suggest that the plan will not include many of the details required by law to be submitted to Congress, such as listing the specific U.S. facilities where detainees would be held and a full cost estimate," the chairman of the House Armed Services Committee wrote in the letter. "To be clear, failing to comply with the law would not advance [the president's] objective of closing the detention facility at GTMO."

A Pentagon spokesman said last month that the Defense Department has completed its portion of the plan, which was being reviewed by the White House.

"We've been working on a plan for several months," Capt. Jeff Davis told Stars and Stripes in late January. "That plan is at the White House now and being considered, and we hope it will soon go to Congress for their consideration."

The facility holds 91 prisoners. The administration has already transferred 16 detainees so far this year, marking an uptick in movement.

Few details of the plan to close the prison have been released, but Defense Secretary Ash Carter said last month that it will include moving those detainees who can't be transferred to a secure facility in the United States.

Last year, defense officials visited military and federal prisons in Colorado, South Carolina and Kansas to evaluate if they'd be suitable for housing Gitmo detainees and help determine costs.

But the effort faced immediate blowback from Capitol Hill. Lawmakers from those states opposed the moves, saying they don't want terrorists in their constituents' backyards.

One senator's objection is actually holding up Obama's pick as the next secretary of the Army from taking over the job. Sen. Pat Roberts, R-Kan., has placed a hold on the nomination of Eric Fanning until he receives assurances that Gitmo detainees won't be housed in his home state.

In addition to naming the facility in the U.S. that will hold detainees who can't be released, the report is also supposed spell out how to handle any prisoners the U.S. may take in future conflicts once the prison is closed.

Asked last week if he thought the prison would close under his tenure as defense secretary, Carter said he didn't know, but he hopes it does so the effort doesn't fall on his successor.

"I don't know whether we'll get it done this year, but to do [it] this way I've described, we need the help and support of Congress. I hope we're getting it," Carter said. "I'm working on it."