He was killed in a drone strike Monday, two officials told CNN.

Rauf and five others were killed, four of them Pakistani militants, said Mohammed Jan Rasoulya, the deputy governor of southern Helmand province. A senior Afghan security source confirmed Rauf's death.

The Washington Post, in a headline last month, called him "the shadowy figure recruiting for the Islamic State in Afghanistan."

The New York Times called him the "militant commander at the center of the concerns in Helmand Province" but said some local Taliban figures "dismiss claims" that he had established "a significant new Islamic State cell in Helmand Province."

He was known to many with the name "Khadim" tacked on to the end of his name.

"Until 9/11, the hard-nosed Khadim commanded (Taliban creator) Mullah Omar's elite mobile reserve force, fighting regime opponents all over Afghanistan," Newsweek wrote of Rauf in a 2011 list of list of most-wanted insurgents. "Arrested and sent to Guantanamo soon after the Taliban's collapse, he was released in late 2007, having convinced his jailers that he wanted only to go home and tend his farm. Escaping from house arrest in Kabul, he fled to Pakistan."

Although the United States does not publicize the names of detainees at Guantanamo, a document posted by WikiLeaks showed that the United States recommended Rauf be "transferred to the control of another country for continued detention" as early as 2004.

In a 2011 hearing of the House Subcommittee on Oversight and Investigations -- part of the Armed Services Committee -- a lawmaker asked about Rauf and another former detainee.

Ed Mornston, director of the Joint Intelligence Task Force of the Defense Intelligence Agency, responded that "there have been instances where detainees who have been transferred from Gitmo have reengaged and have been in the fight and have impacted the lives of U.S. service members. We do track that. I can't discuss that much further in this open session, but we do in fact know that that has happened."

(CNN)