John McDonnell, shadow chancellor for the U.K. Labour Party, second from left, joins Shaker Aamer, second from right, as he is reunited with other Guantanamo detainees in London on Monday. (Luke Macgregor/Reuters)

The United States has repatriated a Saudi prisoner held for nearly 14 years at Guantanamo Bay, Cuba, the Pentagon said Monday, the latest signal of an intensifying effort to shutter the military prison before President Obama steps down.

In a statement, the Pentagon said that Muhammad Abd al-Rahman Awn al-Shamrani, whom U.S. officials describe as an al-Qaeda recruiter and fighter, had arrived in Saudi Arabia, making him the fourth prisoner to be released from Guantanamo this year.

According to military documents made public by WikiLeaks, Shamrani fought with al-Qaeda in Afghanistan in 2001 before being captured in Pakistan and taken to Guantanamo in January 2002. More recent military documents assert that Shamrani, now 40, has followed with interest the rise of the Islamic State, the militant group that controls much of Iraq and Syria, and indicated “possible plans to reengage in terrorist activity.”

In his own written statement from 2015, Shamrani said nothing about wanting to take up arms anew, and he spoke of his desire to open a laundromat in Saudi Arabia. He said he was preparing himself to attend a rehabilitation center run by the Saudi government.

In September, U.S. officials determined that Shamrani could be safely released under Saudi government supervision.

Shamrani’s transfer came as protesters gathered in Washington and London to mark the 14th anniversary of the opening of the military detention center, which now holds 103 prisoners.

In London, the protest was headlined by Shaker Aamer, a British resident whose release in the United Kingdom late last year was cheered by activists there. To supporters in Britain, Aamer’s long detention symbolized the injustice of Guantanamo. Aamer was first cleared for release in 2007 and was never charged with a crime.

Aamer became a leading figure during more than 13 years at Guantanamo, encouraging detainees to launch hunger strikes or other protests.

At Monday’s protest outside the U.S. Embassy, Aamer called on Obama to close the prison immediately and said the U.S. leader could make that happen “overnight.”

But Obama, nearing the end of his presidency, has been unable to fulfill one of his earliest national security goals. Over the past year, military officials have freed prisoners in small bursts; 20 inmates were resettled in 2015. But many other inmates have lingered for years after being cleared for release, in part because of congressional restrictions.

A small number of prisoners are at various stages of a slow-moving military justice process, while still others — those deemed too dangerous to release but who officials say cannot be tried — are expected to be held indefinitely.

Naureen Shah, director of the security and human rights program for Amnesty International in Washington, said that both Congress and the executive branch share blame for not having acted on opportunities to close Guantanamo.

It is unknown whether Obama, facing stiff congressional opposition to bringing prisoners to the United States, will attempt to go around lawmakers and close the prison by executive action. Such a move would be sure to outrage Republicans.

On Sunday, White House Chief of Staff Denis McDonough said that Obama would submit a closure plan to Congress — which has been delayed by internal disagreement among government agencies — and then mull additional actions.

“The president has said from the beginning of this administration that we will close [Guantanamo] because it’s bad for our national security and because it’s too costly,” McDonough told “Fox News Sunday.”

“He feels an obligation to his successor to close that,” McDonough said. “And that’s why we’re going to do it.”

Speaking during a parallel protest outside the White House on Monday, Shah warned that moving prisoners to the United States for indefinite detention would perpetuate human rights violations.

“We don’t want to see the president merely change the Zip code of Guantanamo,” she said.

Also on Monday, Republican Sens. Kelly Ayotte (N.H.) and Lindsey O. Graham (S.C.) called for a halt to all transfers and urged lawmakers to investigate resettlements under the Obama administration, which they said endangered Americans.

Griff Witte in London contributed to this report.