'The threat is real': US authorities treating Tennessee shooting of four Marines as domestic terror attack

The FBI says the gunman who carried out the deadly attacks on two US military centers in Tennessee yesterday had at least two rifles and one handgun, some of which were purchased legally.

Mohammad Youssuf Abdulazeez, 24, killed four Marines and injured three other people yesterday before he died in a shootout with police in Chattanooga.

"He did have at least two long guns, which would be considered rifles or shotguns. And he did have one handgun, that we're aware of," said FBI special agent Ed Reinhold.

"Some of the weapons were purchased legally and some may not have been."

The Kuwait-born Abdulazeez was not wearing body armor, but rather a vest allowing him to carry additional rounds of ammunition, Mr Reinhold said.

The FBI said there was so far "no indication that he was directed or inspired by anyone other than himself," but that investigators were looking at possible links to terror organizations overseas.

That comment came in response to one by senior US lawmaker Michael McCaul, who suggested the attack was inspired by the Islamic State (also known as ISIL or ISIS) group.

"My judgment, in my experience, is that this was an ISIS-inspired attack," Mr McCaul, the chairman of the House Homeland Security Committee, told a press conference in Florida.

Mr McCaul said investigators were trying to piece together Abdulazeez's digital past to see if he had possible links to international terror groups like ISIL or if he had been otherwise swayed by jihadist propaganda on the Internet.

The shooting has revived the specter of lone wolf attacks in the United States - nightmare scenarios because authorities often have no advance warning of a jihadist sympathizer ready to strike.

"What keeps me up at night is the one case we don't know about," Mr McCaul said.

"This is the one that we worried about.

"The threat is real. And it comes from the Internet. This is a new generation of terrorists."