The elusive Mr. Douri, who in recent years has led a Sunni insurgent group that last year aligned with the Islamic State as it took over Mosul, was once declared dead, in 2005, by his own Baath Party. Mr. Douri’s insurgent group is made up of former intelligence officers and elite soldiers from the old Baathist government, and is called the Men of the Army of the Naqshbandia Order, often called J.R.T.N., the group’s initials in Arabic.

The group’s role was said to be important in helping the Islamic State, also known as ISIS or ISIL, seize territory last year. But Men of the Army is believed to have been marginalized as the Islamic State consolidated power through its sheer brutality.

Mr. Douri would have been in his early 70s, and was an interior minister under Mr. Hussein. He was the king of clubs in the deck of cards that was issued to American soldiers in 2003 and carried the faces of the top leaders being sought by American-led military forces.