Four U.S. Marines were killed on Thursday by a gunman who opened fire at two military offices in Chattanooga, Tennessee, before being fatally shot in an attack officials called a brazen, brutal act of domestic terrorism.

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 he was a naturalized American born in Kuwait.

U.S. officials said law enforcement authorities are investigating whether he was inspired by Islamic State or a similar militant group.

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.

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