CHATTANOOGA, Tenn. -- A gunman unleashed a barrage of gunfire at two military facilities Thursday in Tennessee, killing four Marines and wounding two other service members and a police officer, officials told CBS News.

The shooter also was killed. Law enforcement sources told CBS News that the shooting suspect was identified as 24-year-old Mohammod Youssuf Abdulazeez. According to a U.S. law enforcement source, Abdulazeez was born in Kuwait and came to the United States in 1996. It's unclear if and when he became an American citizen.

A U.S. official told CBS News that Abdulazeez was not on law enforcement radar concerning possible terror links or aspirations. The FBI was not aware of him as being any kind of threat.

Authorities identified Muhammad Youssef Abdulazeez as the gunman responsible for the shooting in Chattanooga, Tennessee, July 16, 2015. CBS News

Officials will try to find out if Abdulazeez was motivated or influenced by ISIS.

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The group has been calling for "lone wolf" style attacks in the U.S. on the military and police during the month of Ramadan, which concludes Friday. Over the Fourth of July holiday, federal law enforcement officials warned of the potential for an attack.

"We're going to do an intense look at him to see what his connections are," FBI Special Agent in Charge Ed Reinhold said. "We'll look at his friends, family, associates -- anybody who is associated with him to determine the cause or the reason why he conducted this attack."

At a news conference late Thursday night, Reinhold said so far, investigators had not uncovered evidence linking Abdulazeez with any international terror groups. He said there was "no indication" that anyone else was involved in the attack.

Attorney General Loretta Lynch directed the FBI to take the lead on a "national security" investigation into the shootings, which she called a "heinous attack."

"Today was a nightmare for the city of Chattanooga," Mayor Andy Berke said. "As a city, we will respond to this with every available resource that we have."

U.S. Attorney Bill Killian said officials were treating the attacks as an "act of domestic terrorism," though the FBI's Reinhold said authorities were still investigating a motive. The first shooting happened around 10:45 a.m.; the attacks were over within a half-hour.

Berke said five people died in all, including the gunman.

"The Department of Defense will decide if the names of the deceased Marines will be released," Reinhold said late Thursday.

In addition to those who died, a Marine recruiter was treated at a hospital for a gunshot wound to the leg, the Marine Corps said on its Facebook page. The U.S. Navy said one of its sailors was also among the wounded.

Chattanooga police officer Dennis Pedigo was shot in the ankle, CBS affiliate WDEF reported. Thursday night, he was hospitalized in stable condition after surgery.

"Lives have been lost from some faithful people who have been serving our country, and I think I join all Tennesseans in being both sickened and saddened by this," Gov. Bill Haslam said.

In Washington, President Barack Obama pledged a prompt and thorough investigation into the "heartbreaking" attack and said the White House had been in touch with the Pentagon to make sure military installations are being vigilant.

After the shooting, the Department of Homeland Security said in a statement that it was "enhancing the security posture at certain federal facilities, out of an abundance of caution."

Vice President Joe Biden likewise vowed: "We will get to the bottom of it."

"Their families have already given a lot to the country," he said, "and now this."

Within hours of the bloodshed, law officers with guns drawn swarmed what was believed to be Abdulazeez's house, and two females were led away in handcuffs.

The shootings began at a recruiting center on Old Lee Highway in Chattanooga where five branches of the military all have adjoining offices. A gunshot rang out around 10:30 or 10:45 a.m., said Sgt. 1st Class Robert Dodge, 36, the center leader for U.S. Army recruiting at the center.

"Shortly after that, just a few seconds, the shooter began shooting more rounds. We realized it was an actual shooting," he said.

He and his colleagues then got on the ground and barricaded themselves in a safe place. Dodge estimated there were 30 to 50 shots fired.

He did not see the shooter or a vehicle. The Army recruiting office was not damaged, but doors and glass were damaged at the neighboring Air Force, Navy and Marine offices, he said.

Law enforcement officials told recruiters that the shooter was in a car, stopped in front of the facility, shot at the building and drove off, said Brian Lepley, a spokesman with the U.S. Army Recruiting Command in Fort Knox, Kentucky.

One witness told CBS affiliate WDEF that a man who was in a silver Mustang convertible was "just unloading a large gun on the Naval recruiting office."

The recruiting center sits in a short strip between a Cricket Wireless and an Italian restaurant with no apparent additional security. Nearby, Nicholas Donohue heard a blast of gunshots while working at Desktop Solutions. But he had music playing and wasn't quite sure what the noise had been. He turned off the music and seconds later, a second blast thundered. He took shelter in a back room.

"Even though it knew it was most likely gunfire I heard, you also don't want to believe it's happening in the moment," he said. "Since I didn't see anything, I couldn't be sure."

By the time he emerged, police were cordoning off the area.

Within minutes of that attack, the shooter then opened fire at the Navy Operational Support Center and Marine Corps Reserve Center Chattanooga, about 7 miles away. Reinhold said all of the dead were killed there.

The center sits between the highway and a pathway that runs through Tennessee RiverPark, a popular park at a bend in the Tennessee River northeast of downtown Chattanooga. It's in a light industrial area that includes a Coca-Cola bottling plant.

The two entrances to the fenced facility have unmanned gates and concrete barriers that require approaching cars to slow down to drive around them.

In a statement, Pentagon spokesman Col. Steve Warren addressed security measures at recruiting facilities around the country:

"We have our recruiting centers set up in places easily accessible to the public. For example, a strip mall. So security there is not what you'd see at a Ft. Bragg, a Norfolk Naval Air Station or at Quantico. So this is something we have to face. Security is not as extensive as it would be at a major installation. This is because we have to be in contact with the American public. We're continuing to look into this incident, working very closely with both federal and local law enforcement agencies to determine exactly what happened and if we need to make any adjustments."

Marilyn Hutcheson, who works at Binswanger Glass across the street, said she heard a barrage of gunfire around 11 a.m.

"I couldn't even begin to tell you how many," she said. "It was rapid fire, like pow pow pow pow pow, so quickly. The next thing I knew, there were police cars coming from every direction."

She ran inside, where she remained locked down with other employees and a customer. The gunfire continued with occasional bursts she estimated for 20 minutes.

"We're apprehensive," Hutcheson said. "Not knowing what transpired, if it was a grievance or terroristic related, we just don't know."

They saw dozens of emergency vehicles rush by: bomb teams, SWAT teams, and state, local and federal authorities.