US federal investigators say a gunman who killed four marines in a shooting spree in Tennessee had no known links to international terrorism.

Mohammad Youssuf Abdulazeez, 24, was shot dead by police after he attacked a military recruitment centre and a naval reserve centre in Chattanooga.

Local prosecutors are investigating the attacks - in which three people were also injured - as "domestic terrorism".

However, the FBI says Abdulazeez's motive is unclear.

A police officer and a Marine Corps recruiter were wounded, along with a female sailor who remains in a serious condition in hospital.

President Barack Obama said the attacks were "heartbreaking”.

The attacks unfolded at 10:45 local time (14:45 GMT) on Thursday.

The suspected was seen pulling up in a rented Ford Mustang convertible outside a shopping centre and opening fire, spraying the offices of a military recruiting centre with bullets.

April Grimmett was working at a nearby hair salon, and said she had looked out of a window and saw a man ducking in between cars.

"Shortly after that, we heard the (shots). It was very loud and very fast," she was quoted by CNN as saying. "I could not believe how many bullet holes were in that door. It was insane."

Army spokeswoman Kelli Bland said four Army recruiters had been in the building at the time of the shooting, but no one was injured.

The gunman then left the recruitment centre and drove some six miles (10km) to a naval reserve training centre, officials said.

There he fatally shot the four marines and wounded the sailor before being shot and killed himself in a firefight with police.

Employees of nearby businesses spoke of hearing repeated gunshots as police swarmed the area, before it went silent.

Mohammad Youssuf Abdulazeez: 'Life is short and bitter'

Police have sealed off the area around the house in which the gunman lived as they attempt to piece together what led to the attacks.

Eye-witnesses said two women were led away from the house in the Hixson suburb of Chattanooga in handcuffs.

Abdulazeez was reportedly born in Kuwait and had Jordanian citizenship, but was also a naturalised US citizen who had attended high school in a Chattanooga suburb and graduated from the University of Tennessee with an engineering degree.

Those who knew him have spoken of their shock at hearing of his involvement.

"He was a good kid. ... They're good people," said one neighbour. "I've never had any kind of conflict with them."

US officials said he had not been known to federal law enforcement before the attacks, though he had been arrested locally earlier this year for driving under the influence of alcohol.

"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.

A blog that Abdulazeez wrote was monitored by the SITE Intelligence Group which checks on extremist activity, and could provide a useful lead for investigators.

In a post written on Monday, he said, "This life is short and bitter" and that Muslims should not let "the opportunity to submit to allah... pass you by".

He left this message in his school yearbook: "My name causes national security alerts. What does yours do?"

A statement from the US Department of Homeland Security said it was "enhancing the security posture at certain federal facilities, out of an abundance of caution".