A former al-Qaida courier who endured dehumanizing treatment at CIA black sites before spending more than 15 years in Guantánamo Bay was released in Belize on Thursday, a milestone in President Biden's push to close the high-profile prison but one that underscores he difficulties he will face in doing so.

The resettlement of Majid Khan, who struck a plea deal with U.S. prosecutors in 2012, represented the first time that one of the "high-value" prisoners sent from the secret CIA facilities to the military facility in Cuba in 2006 was freed, his attorneys said.

The nearly year-long delay in releasing Khan, whose 10-year sentence concluded in March, is evidence of the legal and political challenges that Biden must navigate in attempting to finally shut down the facility, which now holds 34 detainees but remains a potent symbol of American excesses in the wake of 9/11.

Free after two decades, Khan apologized for his past. The former Baltimore-area resident pleaded guilty a decade ago to taking part in al-Qaida plots and later provided testimony in other terrorism cases.

"I have been given a second chance in life and I intend to make the most of it," he said in a statement released by his attorneys at the Center for Constitutional Rights (CCR) and Jenner & Block.

The release caps a saga that began in 2003 when Khan, who was born in Saudi Arabia but later emigrated with his family to Maryland, was captured in Pakistan. He then endured beatings, sleep deprivation and other forms of torture at the hands of CIA interrogators at secret overseas prisons before being sent to Guantánamo.

White House press secretary Karine Jean-Pierre said Khan had honored his cooperation commitment. "We remain dedicated to a deliberate and thorough process, focused on responsibly reducing the detainee population," she told reporters Thursday.

Hina Shamsi, director of the American Civil Liberties Union's national security project, called on Biden to take steps to resolve the fates of other prisoners stuck in a years-long military commission process where cases have been bogged down by logistical, security and legal hurdles, many of them related to their harsh treatment by interrogators.

In the meantime, she said, the government's delay in resettling Khan "was calling into question the Biden administration's commitment to resolving the Guantánamo quagmire."

A State Department spokesman, who like some others interviewed for this report spoke on the condition of anonymity under ground rules set by the administration, said officials were considering different options for Guantánamo's remaining prisoners, many of whom now face health challenges that are expected to pose an increasing strain to the isolated facility's capacity in the coming years.

"There are a lot of different moving pieces," the official said.

In addition to the halting progress in the military trials, the White House faces legal constraints to moving prisoners at Guantánamo, including laws that prohibit the transfer of its inmates to the U.S. mainland and barring the repatriation of certain detainees to their home countries.

The challenges echo those confronted by Biden's former boss, President Barack Obama, who laid out closing Guantánamo as an early goal but did not deliver on that pledge. In contrast, Obama's successor, Donald Trump, vowed - but never delivered - to load up the site with new prisoners.

While prosecutors and defense attorneys have taken part in discussions about potential plea deals for other detainees stuck in the labyrinthine commission process, it's unclear whether those will result in plea deals similar to Khan's. The vast majority of the nearly 800 men held at Guantánamo over the years were never charged with a crime.

Khan's case illustrates the glacial pace and opacity that has often characterized proceedings at Guantánamo.

His story is unusual in other ways. By his own account, Khan attempted to adopt an American identity after his family emigrated to the United States from Pakistan in the 1990s. He considered joining the Navy and working for a technology company in Virginia after high school.

But when he returned to Pakistan, he became involved with al-Qaida, meeting with alleged 9/11 mastermind Khalid Sheikh Mohammed and taking part in militant plots, including couriering $50,000 to operatives who carried out a deadly hotel bombing in Indonesia.

As a captive, Khan was imprisoned at multiple CIA facilities abroad, where he endured brutal interrogation tactics including being submerged in ice water until he believed he was drowning, and being suspended naked from the ceiling by his wrists. He said he suffers lasting ailments from that treatment, which also included being hydrated anally after he went on hunger strike.

As part of his deal with prosecutors, Khan pleaded guilty to war crimes including murder and spying. He also vowed that he would never sue the U.S. government over his mistreatment.

"I deeply regret the things that I did many years ago, and I have taken responsibility and tried to make up for them," he said in his statement.

A military jury urged clemency for Khan in part due to the "physical and psychological abuse well beyond approved enhanced interrogation techniques, instead being closer to torture performed by the most abusive regimes in modern history."

Wells Dixon, a CCR attorney who represented Khan, described that letter as "a wake-up call" to the U.S. government that illustrated it would be "unlikely if not impossible" for government prosecutors to obtain a death sentence for Guantánamo prisoners charged in 9/11.

Twenty of the remaining prisoners are now eligible to be repatriated or sent to third countries, a task that presents its own challenges. While numerous detainees have been resettled, some have struggled after years of confinement and abusive treatment.

Khan's lawyers filed a legal challenge in June protesting his continued imprisonment past his allotted sentence, which Dixon called a "clear violation of due process."

A U.S. official said the delay was primarily because of the difficulty involved in finding a country that would take him. Dixon said the government had known since July 2021 that Khan's sentence would conclude the following spring.

"Detainee transfers are often complicated and take time to complete, but there is no excuse for the fact that Majid Khan was held for a year beyond the conclusion of his military commission sentence," he said.

Khan has a wife, and a daughter whom he has never met.