An attack at two military installations in the US state of Tennessee in July that killed four Marines and a sailor was inspired by a "foreign terrorist group," the FBI said.

FBI Director James Comey, whose agency led an investigation along with the Naval Criminal Investigative Service, said late Wednesday that the shooting was a "terrorist" case.

"We have concluded that the Chattanooga killer was inspired by a foreign terrorist organization's propaganda," Comey told reporters.

"We've investigated this from the beginning as a terrorist case."

By designating the attack as having been led by a "foreign terrorist group," the Pentagon was able to award the Purple Heart to the victims and a survivor, an honour extended to those wounded or killed while serving in the US military.

"Although the Purple Heart can never possibly replace this brave Sailor and these brave Marines, it is my hope that as their families and the entire department of the Navy team continue to mourn their loss, these awards provide some small measure of solace," Navy Secretary Ray Mabus said.

Mabus and Comey did not indicate which extremist group might have been linked to the shootings.

"Their heroism and service to our nation will be remembered always."

On July 16, Kuwaiti-born naturalized US citizen Mohammad Youssuf Abdulazeez opened fire at a recruitment center in Chattanooga, then traveled to a Navy and Marine Corps Reserve Center where he continued firing.

The FBI had early on determined that Abdulazeez, 24, had acted alone, while his family spoke of mental issues.

The self-proclaimed Islamic State group, which has seized large swathes of Iraq and Syria, has called on its supporters to target US military personnel.
Source: AFP