The suspect, seen driving an open-top Ford Mustang, first went to a joint military recruiting center in a strip mall and sprayed it with gunfire, riddling the glass facade with bullet holes.
"Everybody was at a standstill and as soon as he pulled away everyone scrambled, trying to make sure everyone was OK," said Erica Wright, who works two doors down from the center.
The 24-year-old gunman then drove off to a Naval Reserve Center about 6 miles away, fatally shooting the four Marines.
Authorities would not say publicly how the gunman died, but the U.S. official said investigators believe Chattanooga police fired the shot that killed him. Chattanooga Chief of Police Fred Fletcher did not speak as to the circumstances of Abdulazeez's death in an update to the press Friday.
Three others were wounded in the attacks, including a police officer reported to be in stable condition and a Marine. The shootings began around 10:45 a.m. and ended about 30 minutes later.
The slain service members were identified Friday by the Marines as Gunnery Sgt. Thomas J. Sullivan of Hampden, Massachusetts; Staff Sgt. David A. Wyatt of Burke, North Carolina; Sgt. Carson A. Holmquist of Polk, Wisconsin; and Lance Cpl. Squire K. Wells of Cobb County, Georgia, who a family spokesman says went by "Skip."
Sullivan was deployed twice during the Iraq war and received two Purple Hearts. Wyatt was deployed during both the Iraq and Afghanistan wars, while Holmquist was deployed to Afghanistan.
NBC News reported that Abdulazeez was a naturalized American who was born in Kuwait.
A U.S. official told The Associated Press that there was no indication Abdulazeez was on the radar of federal law enforcement before the shootings. The official was not authorized to discuss the case and spoke on condition of anonymity.
At a news conference late Thursday, Edward Reinhold, special agent in charge of the FBI's Knoxville, Tennessee, division, said investigators had found nothing that tied the suspect to ISIL or any other armed group. A federal law enforcement official told the Associated Press on Friday that authorities are continuing to search Abdulazeez's computer for evidence pointing at any connection to international armed groups.
"We will continue to investigate this as an act of terrorism until the facts tell us otherwise," Bill Killian, U.S. Attorney General for Eastern Tennessee told the press Friday.
The FBI's special agent in charge in the investigation, Ed Reinhold, told reporters that although the shooting is being "investigated as an act of terrorism," "it would be premature to speculate why the shooter did what he did."
Reinhold said that investigators were still "covering leads coming into our command center," but he added that there was "no imminent threat to anyone in the Chattanooga area."
Reinhold did indicate that “partners overseas” were investigating the incident together with their FBI counterparts.
Another official close to the investigation told Reuters that authorities are investigating the suspect's travel to the Middle East, including at least one trip to Jordan and a possible visit to Yemen.
The SITE Intelligence Group, which tracks global security threats, said that Abdulazeez wrote in an online post Monday that “life is short and bitter” and Muslims should not miss an opportunity to “submit to Allah.” Reuters could not independently verify the writing.
The New York Times, citing unnamed law enforcement officials, reported that his father had been under investigation several years ago, over possible ties to a foreign armed group. His name was later removed from the “Terror Watchlist.”
According to a resume believed to have been posted online by Abdulazeez, he attended high school in a Chattanooga suburb and graduated from the University of Tennessee with an engineering degree.
“I remember him being very creative. He was a very light-minded kind of individual. All his videos were always very unique and entertaining,” said Greg Raymond, 28, who worked with Abdulazeez on a high school television program.
“He was a really calm, smart and cool person who joked around. Like me, he wasn’t very popular so we always kind of got along. He seemed like a really normal guy,” Raymond said.
Al Jazeera and wire services