The U.S. Was Resettling Guantánamo Prisoners. The Hamas Attack Halted Those Plans.

The Biden administration was poised to send about a dozen detainees at Guantánamo Bay to Oman for resettlement last year, but it abruptly halted the secret operation amid questions from Congress about security in the Middle East after Hamas attacked Israel, according to administration officials.

None of the prisoners have ever been charged with crimes, and all of them had been cleared for transfer by national security review panels.

A military cargo plane was already on the runway at Guantánamo Bay ready to airlift the group of Yemeni prisoners to Oman when the trip was called off, people familiar with the military operation said.

Belongings they could take with them had been collected, signaling to the prisoners that they would soon be going. Then the plane flew away empty, and their belongings were returned.

Details of such operations are classified for the security of the U.S. military aircrews that transport the men. But U.S. officials, speaking on the condition of anonymity because detainee movements are considered secret until completed, acknowledged the aborted mission after NBC published an account of it on Monday.

The delayed transfer illustrates the Biden administration’s continuing struggle to find countries willing to resettle and keep watch on the 16 cleared detainees, who are among the 30 men held at Guantánamo. Such deals require diplomacy, participation by the intelligence community and advance notice to Congress.

The United States has long considered Oman to be a strong loyal ally, a peaceful nation 1,500 miles and a landmass away from Gaza. Oman prides itself as a neutral go-between among conflicting regional powers.

On Monday night, Adrienne Watson, a spokeswoman for the National Security Council at the White House, called Oman “a trusted partner” that “cooperates closely with the United States on a range of priorities, including on the rehabilitation of Guantánamo detainees.”

U.S. diplomatic and national security officials reached an agreement with Oman to send the prisoners there last year. But the plan ran into opposition during a closed briefing in Congress in October, virtually on the eve of the transfer. Democrats raised concerns with State Department and intelligence officials about the potential for instability in the Middle East in the aftermath of the Hamas attack on Israel, according to administration officials, who spoke on the condition of anonymity because they were not authorized to discuss the sensitive mission.

The administration agreed to postpone the transfer, and to review the arrangements, a process that two government officials said Monday was continuing. Defense Department policy does not allow the disclosure of a new transfer date until after the prisoners have left.

The Guantánamo prisoners awaiting transfer are from countries considered too unstable or dangerous for repatriation, notably Yemen, requiring the United States to seek the help of allies and partners to receive them for rehabilitation or resettlement.

Across the two-decade history of the detention center in Cuba, about 750 detainees have been transferred through repatriation or resettlement, mostly in secret military operations. A few were sidelined or delayed by diplomatic hitches, a change of leadership in the receiving nation, or U.S. military operational concerns.

In late 2014, for example, a military cargo plane carrying five detainees to Central Asia that left Guantánamo in a long-planned transfer turned back because of a midair mechanical issue. The Pentagon delayed public disclosure of what it considered a sensitive national security operation until a new C-17 cargo plane and crew were sent to the base to pick up the prisoners and a special guard force, and take them to Kazakhstan.

Oman’s rehabilitation program has received 30 detainees from 2015 to 2017. Most were from Yemen, which shares a border with Oman. Many of them have married and now have children there, although it is not known if they have successfully integrated in the society.

Two of the men sent to Oman were citizens of Afghanistan who were repatriated earlier this year, according to the Taliban, after seven years of house arrest.

Matthew Miller, the State Department spokesman, said at the time that the two men were repatriated because security conditions negotiated between the United States and Oman at the time of their transfer, in 2017, had expired.

Ms. Watson said Oman, through its rehabilitation program, “continues to fulfill its humane treatment and security assurances for detainees sent there in recent years, in some cases far longer than anticipated.”

“Given the strength of Oman’s program, we will continue to cooperate closely with Omani officials on these issues moving forward,” she added.

Lawyers for the cleared prisoners have declined to comment on the aborted transfer, or to discuss the mood in Guantánamo’s minimum-security Camp 6 facility, where the men who have been approved for release are segregated from so-called high-value detainees.

The Biden administration’s push to shrink the prison population is part of a renewal of President Barack Obama’s failed pledge to close the operation. But the effort has revived some of the criticism and opposition of the Obama years.

Congress does not have the authority to stop the transfer to Oman. By law, however, it receives a confidential notification of each pending transfer at least 30 days in advance, providing time to raise objections.

Legislation has blocked transfers of detainees from Guantánamo to the United States for any reason.

In addition to Yemen, transfers to Libya, Sudan and Syria have also long been forbidden because those nations are considered too politically unstable or violent for safe repatriation.

Congress added Afghanistan to the list after the administration repatriated a former Afghan militiaman in 2022, with the help of Qatar. A federal court had found the prisoner was unlawfully held, and ordered his release.