LISTEN: Chattanooga Mayor Andy Berke and Chattanooga Police Chief speak at press conference:

UPDATE: The Associated Press has identified the shooter as 24-year-old Muhammad Youssef Abdulazeez.

CHATTANOOGA — (CNN) Four Marines were killed in a shooting at a Naval Reserve center in Chattanooga, Tennessee, on Thursday morning, a senior defense official said.

The suspected gunman is also dead, Chattanooga Mayor Andy Berke 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.”

Witnesses said the shootings unfolded at two sites. First, a man sprayed bullets at the glass doors of a military recruiting center at a strip mall on Lee Highway.

From there, the suspected gunman headed to another location more than seven miles away, the Naval Reserve center on Amnicola Highway.

That’s where the four victims and the gunman were killed, Reinhold said.

The shooting started around 10:45 a.m., he said, and ended within 30 minutes.

A police officer, who was injured when he was shot in the ankle, has been treated at the hospital, Berke 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.

Mayor decries ‘horrific’ shooting

“Horrific incident in our community,” Berke, the mayor, tweeted. “We will release details as they are confirmed. Prayers to all those affected.”

Photos posted on Twitter by media outlets showed police blocking a road and a large number of patrol cars.

Chattanooga State Community College asked people on its main campus to stay inside because of a “confirmed shooting,” the school said on Twitter.

Memorial Hospital in Chattanooga is “on precautionary lockdown” due to the shooting reports, hospital system spokeswoman Lisa McCluskey said.

“We have not received any shooting victims,” McCluskey said. “We are still accepting new patients.” The lockdown, she said, “is just a precautionary measure.”

The-CNN-Wire ™ & © 2015 Cable News Network, Inc., a Time Warner Company. All rights reserved.