Four Marines were killed Thursday in twin shootings at US military centers in the southern state of Tennessee, officials said, opening a probe into what they said was a possible act of "domestic terrorism."

At least two people were injured during the incidents in Chattanooga -- a police officer and a Marine Corps recruiter. The gunman was shot dead, city mayor Andy Berke told reporters.

The incident served as an ugly reminder 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.

"It is incomprehensible to see what happened and the way that individuals who proudly serve our country were treated," Berke said, praising the quick response by law enforcement to prevent further loss of life.

Bill Killian, the US federal prosecutor in that part of Tennessee, said the shootings were being investigated as an "act of domestic terrorism," but officials cautioned that no one should jump to conclusions.

"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 Marine Corps confirmed that all four victims were killed at a Navy and Marine Corps Reserve Center. The second shooting, at the hands of the same man, occurred at a recruitment center several miles away.

- 'All shook up' -

Erica Wright said she witnessed the shooting at the recruiting center through the window of a hair salon two doors down.

"We heard one pop, really loud pop. So we went to the door to see what it was," Wright told CNN. "We saw a guy in a silver Mustang just unloading on the naval recruiting place."

Wright said she watched in horror as the man reloaded his gun and opened fire again. He then backed up his car, pulled up to another part of the recruiting center and started shooting again.

"We're all shook up," she said. "Never expected something like this."

President Barack Obama was briefed about the situation, White House spokesman Eric Schultz said.

Several locations in Chattanooga were placed on lockdown including a local college, area businesses and government offices.

Local media published images of police officers crouching behind their cars with rifles drawn as they sought to take down the shooter.

Bullet holes could be seen in the windows of the recruiting center and shell casings littered the parking lot.

Senator Bob Corker, a former mayor of Chattanooga, said he was "heartbroken."

"This is a difficult day for Tennesseans and our thoughts and prayers are with all affected by this tragedy," he said in a statement.