The probe into the deadly attack on two military facilities in Chattanooga, Tennessee, was being treated as a terrorism investigation, federal authorities said Friday.

“We will continue to investigate it as an act of terrorism until the proof shows us otherwise,” U.S. Attorney Bill Killian said.

FBI Special Agent Ed Reinhold stressed that Thursday’s shooting rampage that left four Marines dead hasn’t been classified as an act of terrorism and that the investigation is ongoing. He also said that authorities don’t believe anyone else is in danger in the Chattanooga community.

Counterterrorism investigators are trying to figure out why a 24-year-old Kuwait-born man who seemingly had a typical suburban American upbringing attacked two U.S. military sites.

Muhammad Youssef Abdulazeez of Hixson, Tennessee, did not appear to have been on the radar of federal authorities before the bloodshed Thursday, officials said, and they were still searching for a motive. Abdulazeez was killed by police.

For months, U.S. counterterrorism authorities have been warning of the danger of attacks by individuals inspired but not necessarily directed by the Islamic State of Iraq and Syria, or ISIS. Officials have said they have disrupted several such lone-wolf plots.

The gunman on Thursday sprayed dozens of bullets at a military recruiting center at a strip mall in Chattanooga, then drove to a Navy-Marine training center a few miles away and shot up the installation. The bullets smashed through windows and sent service members scrambling for cover.

In addition to the Marines killed, three people were wounded, including a sailor who was seriously hurt.

The dead 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. “Skip” Wells of Cobb County, Georgia. Sullivan, Wyatt and Holmquist had served in Iraq, Afghanistan or both.

Reinhold said Abdulazeez had at least two long guns, which could be rifles or shotguns, and at least one handgun.

Earlier, he said investigators have “no idea” what motivated the shooter, but “we are looking at every possible avenue, whether it was terrorism, whether it’s domestic, international, or whether it was a simple criminal act.”

Gen. Ray Odierno, the Army’s top officer, said that security at military recruiting and reserve centers will be reviewed, but that it’s too early to say whether they should have security guards or other increased protection.

Odierno said there are legal issues involved in allowing recruiters to carry guns. And he said the centers need to be open and accessible to the public.

Brandon Elder, who works at a staffing company in the strip mall where the recruiting office is situated, said he heard what he thought was a jackhammer, and then someone shouted, “He’s shooting!”

Elder said he looked out his window onto the parking lot and saw a man in a silver convertible Mustang, a gun propped out the window, spraying bullets into the storefronts.

“He was in front of the recruiting office, just riding up, reversing and driving back,” he said. The barrage lasted maybe three or four minutes, and then the driver took off, he said: “It was crazy, surreal, like a movie. Is this really happening?”

On Friday, Gwen Gott added purple ribbons and a flag to a makeshift memorial taking shape outside the strip mall. It included balloons, piles of flowers and a sign staked into the ground: “You were the son of satan. Now you will answer to the son of God.”

“I love the service. Without them, where would we be as a country?” Gott said.