"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," the Mayor added.

US Attorney Bill Killian said officials were treating the attacks as an 'act of domestic terrorism', though FBI Special Agent in Charge Ed Reinhold said authorities were still investigating a motive.

The first shooting happened around 10:45 am (local time); the attacks were over within a half-hour. Berke said five people died in all, including the gunman. A police officer was shot in the ankle, and others were wounded, he said.

Two US officials, speaking on condition of anonymity to discuss an ongoing investigation, told four US Marines were among the dead.

A Marine recruiter was treated at a hospital for a gunshot wound to the leg, the Marine Corps said on its Facebook page.

"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," Governor Bill Haslam said.

The shootings began at a recruiting center in Chattanooga where five branches of the military all have adjoining offices.

A gunshot rang out around 10:30 or 10:45 am, said Sgt 1st Class Robert Dodge, the center leader for US Army recruiting at the center. 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 neighbouring Air Force, Navy and Marine offices, he said.

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