The US resettled Guantanamo prisoners. The Hamas attack put an end to these plans.

The Biden administration was poised to send about a dozen Guantanamo Bay detainees to Oman for relocation last year, but abruptly halted the covert operation after Congress raised questions about security in the Middle East after Hamas attacked Israel, administration officials said .

None of the prisoners have ever been charged with a crime, and all of them have been cleared for transfer by national security review boards.

A military cargo plane was already on the runway at Guantanamo Bay, ready to ferry the group of Yemeni prisoners to Oman when the trip was aborted, people familiar with the military operation said.

The things they could take with them were collected, signaling to the prisoners that they would soon be leaving. Then the plane flew away empty and their belongings were returned.

Details of such operations are kept secret for the safety of the U.S. military aircrews transporting the men. But U.S. officials acknowledged the aborted mission after NBC published a report about it on Monday, speaking on condition of anonymity because the detainees’ movements are considered secret until they are completed.

The delayed transfer highlights the Biden administration’s ongoing efforts to find countries willing to relocate and monitor the 16 released detainees, who are among 30 men held at Guantánamo. Such deals require diplomacy, intelligence involvement and advance notice to Congress.

The United States has long viewed Oman as a strong loyal ally, a peaceful nation located 1,500 miles and a landmass from Gaza. Oman prides itself as a neutral mediator between conflicting regional powers.

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

U.S. diplomats and national security officials reached an agreement with Oman last year to send the prisoners there, but the plan faced opposition at a closed-door congressional briefing in October, virtually on the eve of the transfer. Democrats expressed concerns to State Department and intelligence officials about the possibility of instability in the Middle East following the Hamas attack on Israel, according to administration officials who spoke on condition of anonymity because they were not authorized to discuss the sensitive mission.

The government agreed to postpone the transfer and review the arrangements, a process that was still ongoing Monday, according to two government officials. Defense Department policy allows a new transfer date to be disclosed only after the prisoners have left.

The Guantanamo detainees awaiting transfer come from countries deemed too unstable or dangerous to repatriate, particularly Yemen, forcing the United States to seek help from allies and partners to accept them for rehabilitation or resettlement.

Over the course of the Cuban internment camp’s two-decade history, approximately 750 prisoners have been transferred through repatriation or resettlement, mostly as part of secret military operations. A few have been sidelined or delayed by diplomatic problems, a change in leadership in the recipient country, or U.S. military operational concerns.

For example, in late 2014, a military cargo plane carrying five detainees bound for Central Asia, leaving Guantanamo as part of a long-planned transfer, turned back in mid-air due to a technical problem. The Pentagon delayed public disclosure of what it considered a sensitive national security operation until a new C-17 cargo plane and its crew were sent to the base to pick up the prisoners and a special guard and take them to Kazakhstan.

Oman’s rehabilitation program received 30 prisoners from 2015 to 2017. Most came from Yemen, which borders Oman. Many of them got married there and now had children, although it is not known whether they have successfully integrated into society.

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

State Department spokesman Matthew Miller 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 that through its rehabilitation program, Oman “continues to meet its humane treatment and security assurances for detainees sent there in recent years, in some cases for much longer than expected.”

“Given the strength of the Omani program, we will continue to work closely with Omani officials on these issues,” she added.

Lawyers for the released prisoners have declined to comment on the aborted transfer or discuss the mood at Guantanamo’s minimum-security facility Camp 6, where the men approved for release are separated from so-called high-value detainees.

The Biden administration’s push to reduce the prison population is part of a renewal of President Barack Obama’s failed promise to shut down operations. But the effort has revived some of the criticism and opposition from the Obama years.

Congress has no authority to stop the transfer to Oman. However, the law requires it to receive confidential notice of any pending transfer at least 30 days in advance to allow time to raise objections.

Lawmakers have blocked the transfer of Guantánamo detainees to the United States for some reason.

In addition to Yemen, renditions to Libya, Sudan and Syria have long been banned because these countries are considered too politically unstable or violent for safe repatriation.

Congress added Afghanistan to the list after the administration, with help from Qatar, repatriated a former Afghan militia officer in 2022. A federal court found that the prisoner was being held unlawfully and ordered his release.