WASHINGTON - Shootings at two military facilities in Chattanooga, Tennessee, left four US marines dead and three others wounded Thursday. The gunman was also killed later, authorities said.

According to local police, the two shooting incidents, which lasted for about 30 minutes, happened around 11:00 am, local time, at a military recruiting center and the Naval Reserve Center about 10 kilometers apart. The shooter reportedly shot while driving in an open-top Mustang.

Law enforcement officers detain a woman as they surround a house in Hixson, Tenn, July 16, 2015. [Photo/IC]

The Federal Bureau of Investigation (FBI) had already identified the suspect as 24-year-old Muhammad Youssef Abdulazeez. US President Barack Obama said a full investigation was under way with the FBI in the lead.

"We've also been in contact with the Department of Defense to make sure that all our Defense facilities are properly attentive and vigilant as we sort through exactly what happened," said Obama in a statement, adding that initial investigation showed that the gunman acted alone.

Earlier Thursday the FBI said it had already started its investigation into what local officials described as "an act of domestic terrorism."

"We will treat this as a terrorism investigation until we determine it was not," said Ed Reinhold, an FBI special agent in charge of the investigation, at a press conference. "We have not determined if it was an act of terrorism or a criminal act."