Update as of 5:40 a.m. EDT: One of the four Marines killed in Thursday’s shooting in Chattanooga, Tennessee, has been identified as Gunnery Sgt. Thomas Sullivan, of Springfield, Massachusetts, according to media reports, which cited his family. However, the Department of Defense has not yet released the names of the four victims.

The 40-year-old, who had been in the Marine Corps since 1997, was gunned down by suspect Muhammad Youssef Abdulazeez in an attack on two military sites.

“There’s no Marine you would want that was better in combat than him,” Sullivan’s friend Josh Parnell, of Chicago, told Oak Lawn Patch, a local news website.

Original story:

Authorities investigating the shooting at Chattanooga, Tennessee, have not found any evidence to link suspect Muhammad Youssef Abdulazeez to an international terrorist group, the FBI said, at a news conference Thursday night. The gunman opened fire at two military sites Thursday morning, leaving four Marines dead and wounding several others.

FBI agent Ed Reinhold reportedly said that at this point investigators believe the suspect was the only gunman involved in the attack. The shootings began around 10:45 a.m. EDT when the shooter fired from inside his vehicle while driving by a military recruiting center at a strip mall, Reinhold said, according to the Associated Press (AP). He added that the shooter then drove to another U.S. military site where he got out of his vehicle before fatally shooting the marines.

"There is no indication at this point that anybody else was involved," Reinhold reportedly said. “Obviously, we're still at the beginning of this investigation... We will explore any possibility and that includes whether or not anyone else was involved."

However, police officers detained two women after raiding a home near Chattanooga believed to belong to the shooting suspect.

Authorities are reportedly looking for a motive behind the attack on the two military facilities that also killed the gunman. Reinhold reportedly said that an autopsy would determine the cause of the suspect's death, adding that the Department of Defense will decide whether to release the names of the marines killed in the shootings.

In a news conference Thursday night, U.S. Attorney Bill Killian reportedly said that authorities do not believe that there are any threats to the general public following the fatal shooting.

The investigation into the shooting is "an ongoing extensive and expansive investigation with federal, state and local agencies, headed by the FBI," Killian said, according to AP. "As far as we know, at this juncture there are no safety concerns for the general public."

On Thursday, President Barack Obama sent out his “deepest condolences” to the victims’ families, and said that he was briefed about the shootings and has been in touch with the Department of Defense to make sure military facilities are on alert.