NAIROBI, Kenya — A young man from Alabama who traveled to Somalia and became an infamous Islamist militant, commanding guerrilla forces and earning a $5 million American bounty on his head, was believed to have been killed by his former extremist allies on Thursday, according to news reports and Islamist Web sites.

The jihadist, Omar Hammami, known for his rap-infused propaganda videos for the Shabab, a brutal Islamist group in Somalia, was reported killed in an ambush on Thursday morning. If true, his death would bring to a close one of the more unusual chapters in more than two decades of fighting in the Horn of Africa.

But Mr. Hammami, also known by the nom de guerre Abu Mansoor Al-Amriki, “the American,” has been declared dead before, only to resurface alive.

There is little question that Mr. Hammami has been on the run from his former comrades. His recent troubles brought to the surface rifts within militant circles in Somalia, particularly between foreign fighters and Somalis. In a Twitter message in April, Mr. Hammami said the group’s leader had “gone mad” and was “starting a civil war.”