He was a Taliban commander captured by the United States and held at Guantanamo Bay. But he was let go and returned to Afghanistan. Mullah Abdul Rauf went on to become a recruiter for ISIS in Afghanistan.

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.