FOUR Marines were killed in twin shootings at US military centres in the southern state of Tennessee in what officials 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.

A US official speaking on condition of anonymity identified the gunman as 24-year-old Muhammad Youssef Abdulazeez of Hixson, Tennessee, though the spelling of his first name was in dispute, with federal authorities and records giving at least four variations.

The official said Abdulazeez was believed to have been born in Kuwait. It was unclear whether he was a US or Kuwaiti citizen.

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.

Reinhold said the shooter was an area resident and that the FBI would use every means possible to “determine the cause or the reason why he carried out this attack.”

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

The gunman

Little was immediately known about the shooter.

NBC News and CNN reported that he was a naturalised US citizen, originally from Kuwait.

Local news network WRCB said Abdulazeez graduated from the University of Tennessee in 2012 with a degree in engineering.

The Chattanooga Times Free Press said he was a soil engineering specialist working for the city.

A woman who attended Red Bank High School with Abdulazeez said he was a quiet kid, but well-liked.

“He was friendly, funny, kind,” Kagan Wagner told the Chattanooga Times Free Press. “I never would have thought it would be him.” “They were your average Chattanooga family,” Wagner said.

In an apparent blog post written on Monday, he said Muslims should not let “the opportunity to submit to allah ... pass you by,” according to the SITE Intelligence Group, which monitors extremist activity on social media.

Berke told CNN there had not been “any indication” that an attack was imminent.

‘All shook up’

Erica Wright said she witnessed the shooting at the recruiting centre 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 centre 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 centre 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.