Official plans to shutter Guantanamo Bay and move its remaining prisoners to the U.S. mainland before the end of President Barack Obama's final term are underway, Secretary of Defense Ash Carter announced. While the desire to shut the detention center on Cuba has existed since Obama took office in 2009, issues over where to send the prisoners and concerns a Republican-led Congress wouldn’t agree have so far hampered plans.

Carter said Thursday in Miami not everyone at the Guantanamo prison “can be safely transferred to another country, so we need an alternative,”Defense One reported. “I have therefore framed for the president a proposal to establish an alternative location. That plan will propose bringing those detainees to an appropriate, secure location in the United States.”

Carter made it clear in his announcement no specific site had yet been chosen, but there were a number under consideration that would house the very worst detainees, some of whom have been at the prison since it opened in 2002. Many have faced no charges. 

A senior administration official told Defense One Obama received the proposal last month and security officials were close to deciding on a location for a detention site in the U.S. 

Just before Carter spoke in Miami, 10 Yemeni detainees from the prison were transferred to Oman, reducing the prisoner population at Guantanamo to 93 — the first time the prison has had fewer than 100 prisoners since it opened — a figure that ought to placate congressional fears closing the facility would release potential terrorists who could then target the U.S.

However, there is strong opposition in Congress to closing the facility. Sen. Kelly Ayotte, R-N.H., said in a statement Wednesday any Obama administration decision “to transfer a large number of Yemeni detainees from Guantanamo to Oman would represent a thinly veiled attempt to undercut the will of Congress and would further endanger the American people.”

It's expected a further three detainees are to leave the prison in early 2016, the New York Times reported.