CHATTANOOGA, Tennessee — In an attack officials called a brazen act of domestic terrorism, a gunman opened fire at two military offices in Chattanooga on Thursday morning, killing four Marines before being fatally shot.

The FBI named the suspect as Mohammod Youssuf Abdulazeez, 24, but said it was too early to speculate on a motive for the rampage, which comes at a time when U.S. military and law enforcement authorities are increasingly concerned about the threat posed by “lone wolves” to domestic targets.

“We are treating this as an act of domestic terrorism,” Bill Killian, the U.S. attorney for the Eastern District of Tennessee, said earlier, adding that no official determination of the nature of the crime had yet been made and the Federal Bureau of Investigation has not ruled anything out.

“While it would be premature to speculate on the motives of the shooter at this time, we will conduct a thorough investigation of this tragedy and provide updates as they are available,” the agency said in a statement.

NBC reported that Abdulazeez was a naturalized American born in Kuwait, and U.S. officials said law enforcement authorities were investigating whether he was inspired by Islamic State or a similar militant group.

According to a resume believed to have been posted online by Abdulazeez, he attended high school in a Chattanooga suburb and graduated from the University of Tennessee with an engineering degree.

“I remember him being very creative. He was a very light-minded kind of individual. All his videos were always very unique and entertaining,” said Greg Raymond, 28, who worked with Abdulazeez on a high school television program.

“He was a really calm, smart and cool person who joked around. Like me he wasn’t very popular so we always kind of got along. He seemed like a really normal guy,” Raymond said.

Mary Winter, president of the Colonial Shores Neighborhood Association, said she had known Abdulazeez and his family for more than 10 years and was stunned at the crime.

“We’re all shocked and saddened,” Winter said. “He never caused any trouble. We can’t believe that this happened.”

President Barack Obama offered his condolences to the victims’ families and said officials will be prompt and thorough in getting answers on the shootings.

“It is a heartbreaking circumstance for these individuals who have served our country with great valor to be killed in this fashion,” he said in a statement from the Oval Office.

The Department of Homeland Security was stepping up security at certain federal facilities and supporting the FBI investigation, DHS Secretary Jeh Johnson said in a statement.

Chattanooga is a city of about 173,000 people along the Tennessee River in the southeastern Tennessee.

The suspect, seen driving a Ford Mustang convertible, is believed to have first gone to a joint military recruiting center in a strip mall and sprayed it with gunfire, riddling the glass facade with bullet holes.

“Everybody was at a standstill and as soon as he pulled away everyone scrambled trying to make sure everyone was OK,” said Erica Wright, who works two doors down from the center.

Armed forces recruiting centers are often located in shopping centers and other prominent places.

The gunman then drove off to a Naval Reserve Center about 6 miles away, fatally shooting the four Marines before being shot and killed in a firefight with police.

At least three people were wounded in the attacks, including a Marine and a Navy sailor who is in critical condition, according to the hospital. One of those hurt was a police officer who was in stable condition.

Police blocked access to the street where the suspected gunman lived in an upscale suburb. Only residents with photo IDs were allowed to pass and all cars coming and going were stopped.

Local media said memorial services for the victims would be held in various Chattanooga churches Thursday night.

“We condemn this horrific attack in the strongest terms possible,” said Nihad Awad, national director of the Council on American-Islamic Relations.