HIXSON, Tenn. — He played whiffle ball in a suburban neighborhood of swim meets and gently sloping lawns. He was a young man who was polite, who sometimes drove too fast, who was arrested on a drunken-driving charge. He was a hardened mixed-martial arts fighter who kept a blog where he mused about submitting to Allah.

And on Thursday, the authorities say that Mohammod Youssuf Abdulazeez, 24, killed four Marines and wounded at least three other people. He himself died in what the United States attorney here called a potential act of “domestic terrorism.”

According to a federal official, Mr. Abdulazeez was a Kuwaiti-born Jordanian who became a naturalized United States citizen. According to The Chattanooga Times Free Press, he graduated from Red Bank High School, whose quotation on his graduation yearbook page was: “My name causes national security alerts. What does yours do?”

His neighbors here in Hixson, the suburb of Chattanooga where he grew up, said that he seemed to have been an all-American boy, handsome and polite, normally in a T-shirt and jeans.