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.
For more than a decade, Mr. Abdulazeez, his parents and his sisters had made their home in a subdivision called Colonial Shores, just across the Tennessee River from Chattanooga. It is a movie-set vision of American suburbia, almost surreally well-tended, with handsome middle-class homes of a late-1970s vintage.
Dean McDaniel, 59, who lived two houses down from the Abdulazeez family on Colonial Way Circle, said he had known Mr. Abdulazeez for the last dozen years or so, when the family moved into the neighborhood. Mr. McDaniel said that two of Mr. Abdulazeez's sisters used to babysit for the children of Mr. McDaniel when they were younger. Mr. Abdulazeez would sometimes visit his sisters while they were there.
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He remembered Mr. Abdulazeez and his siblings as well behaved and polite. You could tell, he said, that they had strict parents and a structured lifestyle.
The girls and the mother wore head scarves, he said, while the son dressed in typical American garb: T-shirts, jeans, shorts. The parents spoke with an accent, the children less so. Mr. McDaniel said he knew they were Muslim, although he did not know where they were from.
He remembered the young Mr. Abdulazeez playing whiffle ball and other sports outside with the other neighbors' children.
He said Mr. Abdulazeez grew up big, more than six feet tall, muscular and athletically built. The last time Mr. McDaniel remembers seeing him was about two or three years ago, he said.
Chet Blalock, the owner of the Blalock International Mixed Martial Arts Academy in Chattanooga, said Mr. Abdulazeez wrestled in high school. He said some of his students had trained with Mr. Abdulazeez, though he did not know him well himself.
"He was a tough guy, he wouldn't tap out; he elected to pass out," Mr. Blalock said.
A résumé that Mr. Abdulazeez posted on the Internet shows that he received an engineering degree from the University of Tennessee-Chattanooga in 2012 and interned with the Tennessee Valley Authority, learning to manage electrical power systems.
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Another neighbor, W. Keith Clingan III, said that about six or eight months ago, Mr. Abdulazeez came roaring up the hill in a Toyota Camry with no hubcaps, driving through a yield sign in front of Mr. Clingan's home.
Mr. Clingan, 56, an owner of an auto body and glass company, asked the young man to stop. It was the second time that he had seen Mr. Abdulazeez fail to heed the yield sign. "I told him to slow down and I said, 'Do you have a problem with that?' He said, 'No, sir, I'll slow down.' "
After that, Mr. Clingan said, Mr. Abdulazeez would always give a friendly wave as he drove by.
Mr. Abdulazeez maintained a blog where he posted about Islam, according to the SITE Intelligence Group, which tracks international terrorist groups. A July 13 post stated that "life is short and bitter" and that Muslims should not let "the opportunity to submit to Allah ... pass you by."
Photographs posted on family Facebook pages show the once-clean-cut student recently grew a beard. A Hamilton County jail booking report posted on Chattanoogan.com shows that he was arrested on a charge of driving under the influence, a first offense.
In the last two or three months, Mr. Abdulazeez had begun showing up rather regularly at Friday Prayer at the Islamic Society of Greater Chattanooga, a large mosque and cultural center, said Dr. Azhar S. Sheikh, a founding member of the center's board.
Dr. Sheikh said that he showed no signs of extremism.
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He said the family regularly worshiped at the mosque. He thought the young man had stopped attending services because he had moved away, and he assumed that Mr. Abdulazeez was attending again because he was back in town and staying with his parents.
The mosque was in mourning for the Marines he is accused of killing. To honor them, Dr. Sheikh said, the center canceled its Eid al-Fitr celebration planned for Friday. Eid is normally a joyous affair, with children receiving gifts. About 1,000 people normally attend.
"We have canceled out of respect and remembrance for our fallen Marines," he said.
Mary Winter, 32, the president of the Colonial Shores Neighborhood Association, said she knew the family, and that they were known as good and conscientious neighbors.
"This has been a huge shock in our neighborhood and our community," she said. "Our hearts go out to the Marines who were killed, but our hearts also go out to the family."
Reporting for this article was contributed by Matt Apuzzo, Eric Schmitt, Matthew Rosenberg and Gardiner Harris from Washington; Jess Bidgood from Boston; and Richard Pérez-Peña and Dave Philipps from New York. Jack Begg and Elisa Cho contributed research.