(CNN) -- Four Marines were killed when a gunman attacked two military sites in Chattanooga, Tennessee, on Thursday, the U.S. Navy said.
The suspected shooter, 24-year-old Mohammod Youssuf Abdulazeez, is also dead, the FBI said.
Investigators "have not determined whether it was an act of terrorism or whether it was a criminal act," Ed Reinhold, FBI special agent in charge, told reporters. "We are looking at every possible avenue, whether it was terrorism -- whether it was domestic, international -- or whether it was a simple, criminal act."
U.S. Attorney Bill Killian earlier told reporters that authorities were treating the shooting as an "act of domestic terrorism."
Abdulazeez was born in Kuwait and had Jordanian citizenship, two law enforcement officials said. He was a naturalized U.S. citizen, one official said. Abdulazeez studied at the University of Tennessee in Chattanooga, a local law enforcement official said.
A family member's Facebook page said he graduated with a degree in electrical engineering in 2012. He had a prior arrest for DUI.
The shootings unfolded at two sites over 30 minutes, Reinhold said.
Abdulazeez started spraying bullets at the glass doors of a strip mall military recruiting center around 10:45 a.m. ET, witnesses said.
From there, he headed to another location more than seven miles away, an operational support center operated by the U.S. Navy.
That's where the four victims and the gunman were killed, Reinhold said.
At least three people were injured, officials said, including a police officer and a military service member.
The officer, who was injured when he was shot in the ankle, has been treated at the hospital, Chattanooga Mayor Andy Berke said. A military service member was also among the wounded, the Defense Department said.
'It was very loud and very fast'
"Today was a nightmare for the city of Chattanooga. ... We had someone viciously attack, at two different locations, people who proudly serve our country," Berke said.
Gina Mule, a server at a restaurant, said she saw a man who'd been in a silver convertible Mustang, firing a "high-powered rifle" at the recruiting offices at about 10:50 a.m.
She said she first heard "Pow, pow, pow!" She then went to a window, through which she said she saw the man firing shots.
"I don't know how many shots he fired, but it was a lot," she said.
A second witness, April Grimmett, said she was working at a hair salon near the recruiting center when she looked out a window and saw a man ducking in between cars.
"Shortly after that, we heard the (shots). It was very loud and very fast," Grimmett said, without saying what happened to the man she had seen.
A photo that she took shows glass doors damaged by the gunfire.
"I could not believe how many bullet holes were in that door. It was insane," she said.
Fred Wright, a salesman at a nearby auto parts store, told CNN that after he heard gunshots, three people ran into the shop, hollering they thought "someone was shooting at them."
Wright said he called 911.
"Sounded like it was 30, 40 shots to me," Wright said, adding that he never saw a shooter.
CNN's Joshua Gaynor, Pam Brown, Barbara Starr, Evan Perez, Shimon Prokupecz, Devon Sayers, Ashley Fantz and Victor Blackwell contributed to this report.