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A gunman has killed four US Marines and injured several others at two US Navy buildings in Chattanooga, Tennessee.

A local district lawyer said the shootings were being investigated as an "act of domestic terrorism".

The gunman, who was shot dead, was named as 24-year-old Mohammad Youssuf Abdulazeez by the FBI and local reports said he was born in the Middle East.

President Barack Obama said the attack was "heartbreaking" and said the suspect appeared to be a "lone gunman".

Abdulazeez is believed to have been born in Kuwait, but has lived in the US for several years.

He was arrested earlier this year in Chattanooga for driving under the influence of alcohol.

A spokesman for the University of Tennessee at Chattanooga said a student with the same name graduated in 2012 with a degree in engineering, according to local media reports.

The Chattanooga Times newspaper reported that he went to a local high school and competed on the wrestling team.

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

Analysis: By Gary O'Donoghue, BBC News, Tennessee

The investigation is still at an early stage, but it appears that Muhammad Youssef Abdulazeez was acting alone. What made him do what he did will be the subject of intensive inquiries that will delve deep into his past and that of his friends and family, and into his state-of-mind.

But lone wolves, as such men are often described, are the hardest to stop.

Both the president and the head of the FBI have recently underlined this problem and called on local communities to be vigilant for the signs of any radicalisation.

It is also hard for the authorities to protect all potential targets. The first location at which Abdulazeez began firing was an army recruitment office, in the middle of a strip mall - flanked by a mobile phone shop and an Italian restaurant - in other words, firmly within the local community. Many will not want the military to completely retreat behind barbed wire and concrete barriers.