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.
Abdulazeez recently traveled to Kuwait and Jordan, CBS News has learned. Investigators will need to track down who he visited, stayed with, met with and communicated with while overseas, officials told CBS News. That will bring in police and intelligence services of Kuwait and Jordan - both are U.S. allies and cooperation is expected.
Investigators removed computers and other devices believed to have been used by Abdulazeez, CBS News has learned. They are en route to the FBI lab in Quantico, Virginia.
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.
So far, there is no immediate link between the rampage and ISIS or other terrorist groups, but officials stress this is very early in the investigation. The forensic search of computers, phones and online activity may take some time.
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.
Friday night, more than a thousand people gathered at a Baptist church for an interfaith memorial service, to mourn the victims and show support for their families, CBS affiliate WDEF reported.
"What are we supposed to do when a city is marked by that kind of trauma, when a whole city feels grief? What are we supposed to do? We are supposed to seek the peace of the city in which we are called," Governor Bill Haslam said.
Pakastani-born Dr. Mohsin Ali, a psychiatrist, told the audience that Chattanooga's Muslim community was appalled by Abdulazeez's actions. Referring to Abdulazeez as "the murderer," Ali called the shooting "cowardly and cruel."
He also asked all Muslims in attendance who "pledge your allegiance to this city and this country, to this community" to "stand up and be recognized." Dozens of Muslims then stood, many wiping tears from their eyes as the hometown crowd cheered and applauded them.
Reinhold said Abdulazeez had at least two long guns, which could be rifles or shotguns, and at least one handgun. While some of the weapons were purchased legally, some may not have been, he said.
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."
As investigators probed the gunman's background, reports surfaced that Abdulazeez had failed a background check in May 2013 for a job at a Ohio nuclear plant. However, Todd Schneider, a spokesman for FirstEnergy Corp., told CBS News the report was false.
While Schneider acknowledged that Abdulazeez was let go after a period of conditional employment, he was dismissed because "he did not meet minimum requirements for ongoing employment."
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.
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.