FBI warns against 'speculation' over Tennessee shooter's motives

The FBI has warned against speculation about the motives of the gunman in the Tennessee shootings that left four marines dead.

"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.

The attack was branded as an act of "domestic terrorism" by one official shortly after the incident in the city of Chattanooga, and security has been stepped up at federal facilities.

The Department of Homeland Security described the security precautions as an "abundance of caution" in the wake of the attack.

Further details of the attacks have also emerged:

The suspected shooter was seen driving an open-top Ford Mustang, is believed to have first gone to a joint military recruiting center in a shopping centre and sprayed it with gunfire, riddling the glass facade with bullet holes.

The gunman then drove off to a Naval Reserve Center about 6 miles (10 km) away, where he fatally shoot four marines, before being shot and killed in a firefight with police.

The shootings began at about 10.45 am local time (3.45pm UK time) and the attack lasted about half an hour.

Chattanooga Police Chief Fred Fletcher told a news conference many officers had converged to engage the "brazen criminal" and that one officer had been injured during the shootout.