A 24-year-old gunman opened fire at two US military centers in the southern state of Tennessee on Thursday, killing four Marines in what officials said was a possible act of "domestic terrorism."

A 24-year-old gunman opened fire at two US military centers in the southern state of Tennessee on Thursday, killing four Marines in what officials said was a possible act of "domestic terrorism."

The gunman -- identified by the FBI as 24-year-old Mohammod Youssuf Abdulazeez -- was shot dead. Three people were injured, including a police officer and a Marine Corps recruiter.

US President Barack Obama called the shootings "heartbreaking" and asked Americans to pray for the relatives of the victims.

Bill Killian, the US federal prosecutor in that part of Tennessee, said the shootings were being investigated as an "act of domestic terrorism."

"We are looking at every possible avenue -- whether it was terrorism, whether it was domestic, international or whether it was a simple criminal act," FBI special agent Ed Reinhold said.

The FBI said in a later statement that it was "premature to speculate on the motives of the shooter at this time," pledging to conduct a "thorough investigation of this tragedy."

Little was immediately known about the shooter.

WRCB news said Abdulazeez graduated from the University of Tennessee in 2012 and showed footage of law enforcement officers preparing to search his suburban home.

The Marine Corps confirmed that all four victims were killed at a Navy and Marine Corps Reserve Center in the late morning. About 40 minutes earlier, the gunman had opened fire at a recruitment center several miles away.

The Department of Homeland Security said it had ordered security be stepped up at "certain federal facilities, out of an abundance of caution."

The incident reminds of other deadly shootings at US military installations, including a 2009 rampage at Fort Hood that left 13 dead and a 2013 attack at the Navy Yard in Washington that left 12 dead.