Four Marines have been killed in twin shootings at US military centres in the southern state of Tennessee, officials say, opening a probe into what they said was a possible act of "domestic terrorism."
At least two people were injured on Thursday (local time) 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 centre several miles away.
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.