According to U.S. Defense Department records, all the Yemeni men are former al-Qaeda members, although many of them dispute the U.S. accounts of their backgrounds. Before arriving at Guantánamo between 2002 and 2004, several were held at secret overseas CIA prisons known as black sites, where torture was common.

Eventually, the U.S. government determined they no longer posed a significant enough threat to justify their continued imprisonment. However, several of the transfers are contingent upon "robust" security assurances from Oman, such as monitoring, travel restrictions and "integration support." Oman has said it will help the men find housing, jobs and other support systems aimed at letting them rebuild their lives, and in some cases will reunite them with family members.

Three other Guantánamo inmates -- a Libyan, a Somalian and a stateless Rohingya -- have been approved for transfer but remain imprisoned. The Libyan and Somalian cannot be returned to their homelands because those countries, like Yemen, are on Congress's no-transfer list due to security concerns, so the U.S. must find other countries in which to resettle them.

The Iraqi prisoner whose transfer is in the works has pleaded guilty to war crimes charges and the U.S. wants to send him to a prison in Iraq. However, the man recently filed a federal lawsuit seeking to stop his transfer because he says the Iraqi prison could subject him to abuse and inadequate medical care; he has a disabling spine condition.

The U.S. prison and military court at Guantánamo were set up to house and prosecute suspected foreign terrorists after the 9/11 attacks. The first prisoners arrived in January 2002 and about 780 detainees eventually passed through the facility, most of them never charged yet held for years. Guantánamo's supporters say it is a security necessity. Critics call it a moral stain on America.

In addition to quelling human rights complaints, transferring prisoners to other countries represents a huge financial savings to the U.S., which spends more than a half-billion dollars a year on Guantánamo's court and prison. Each inmate held there costs American taxpayers an estimated $15 million a year, compared to about $80,000 annually per inmate at a U.S. supermax facility.

Still, some members of Congress, both Republicans and Democrats, object to releasing Guantánamo prisoners on grounds that they could be a security risk, especially with the ongoing upheaval in the Middle East.

A State Department official told NPR that Monday's transfer "was the culmination of years of diplomatic effort and it did advance the Biden-Harris administration's goal of responsibly reducing the detainee population" at Guantánamo.

The 15 remaining prisoners are:

seven men facing charges, including alleged 9/11 mastermind Khalid Sheikh Mohammed, who has agreed to a plea deal

three uncharged men not approved for transfer

three uncharged men approved for transfer but waiting for the U.S. to find countries to take them

two men who have been convicted

The 11 Yemeni men transferred Monday are:

1. Khaled Ahmed Qassim

captured: December 2001

arrived at Guantánamo: May 2002

cleared for release: July 2022

2. Uthman Abdul al-Rahim Muhammed Uthman

captured: December 2001

arrived at Guantánamo: January 2002

cleared for release: May 2021

3. Moath Hamza Ahmed al-Alwi

captured: December 2001

arrived at Guantánamo: January 2002

cleared for release: December 2021

4. Zuhail Abdo Anam Said al-Sharabi

captured: February 2002

arrived at Guantánamo: May 2002

cleared for release: November 2021

5. Hani Saleh Rashid Abdullah

captured: September 2002

arrived at Guantánamo: October 2002

cleared for release: October 2020

6. Omar Mohammed Ali al-Rammah (alternative name: Zakaria Al-Baidany)

captured: April 2002

arrived at Guantánamo: May 2003

cleared for release: December 2021

7. Tawfiq Nasir Ahmed al-Bihani

captured: late 2001 or early 2002

arrived at Guantánamo: February 2003

cleared for release: 2010

8. Sanad Yislam al-Kazimi

captured: January 2003

arrived at Guantánamo: September 2004

cleared for release: October 2021

9. Hassan Mohammed Ali Bin Attash

captured: September 2002

arrived at Guantánamo: September 2004

cleared for release: April 2022

10. Abdu Ali al-Hajj Sharqawi 

captured: February 2002

arrived at Guantánamo: September 2004

cleared for release: June 2021

11. Abdulsalam al-Hela (alternative name: Abd al-Salam al-Hilah)

captured: September 2002

arrived at Guantánamo: September 2004

cleared for release: June 2021