(CNN) A top al Qaeda in the Arabian Peninsula leader -- who a few years ago was in a U.S. detention facility -- was among five killed in an airstrike in Yemen, the terror group said, showing the organization is vulnerable even as Yemen appears close to civil war.

Ibrahim al-Rubaish died Monday night in what AQAP's media wing, Al-Malahem Media, called a "crusader airstrike." The Al-Malahem Media obituary characterized al-Rubaish as a religious scholar and combat commander.

A Yemeni Defense Ministry official and two Yemeni national security officials not authorized to speak on record confirmed that al-Rubaish had been killed, but could not specify how he died.

Al-Rubaish was once held by the U.S. government at its detention facility in Guantanamo Bay, Cuba. In fact, he was among a number of detainees who sued the administration of then-President George W. Bush to challenge the legality of their confinement in Gitmo.

He was eventually released as part of Saudi Arabia's program for rehabilitating jihadist terrorists, a program that U.S. Sen. Jeff Sessions, R-Alabama, characterized as "a failure." In December 2009, Sessions listed al-Rubaish among those on the virtual " 'Who's Who' of al Qaeda terrorists on the Arabian peninsula ... who have either graduated or escaped from the program en route to terrorist acts."

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The United States has been active in Yemen, working closely with governments there to go after AQAP leaders like al-Rubaish. While it was not immediately clear how he died, drone strikes have killed many other members of the terrorist group.

Yemen, however, has been in disarray since Houthi rebels began asserting themselves last year. The Shiite minority group even managed to take over the capital of Sanaa and, in January, force out Yemeni President Abdu Rabu Mansour Hadi -- who had been a close U.S. ally in its anti-terror fight.

Hadi still claims he is Yemen's legitimate leader, and he is working with a Saudi-led military coalition to target Houthis and supporters of former President Ali Abdullah Saleh.