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".
The suspect 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 named graduated in 2012 with a degree in engineering, according to local media reports.
FBI agent Ed Reinhold, who is leading the investigation, said the first shooting occurred at about 10.45am (local time) at a US Navy recruitment centre in the east of the city.
After opening fire on the building, the gunman then fled the scene in a Ford Mustang and was pursued by Chattanooga police, Mr Reinhold told reporters.
He was shot dead after a gunfight at a US Navy reserve centre about 10 kilometres away on Amnicola Highway.
The US Marines confirmed in a statement that there were "four Marine fatalities" at the Navy and Marine Corps Reserve Centre in Chattanooga.
They said one Marine Corps recruiter was wounded but was later released from hospital. Two other people are believed to have been treated for injuries.
"This is a sad day for the United States. These service members served their country with pride," Bill Kilden, the federal prosecutor for eastern Tennessee, said.
"We are investigating this as an act of domestic terrorism," he told reporters.
But other officials expressed caution at jumping 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," Mr Reinhold said.
City Mayor Andy Berke tweeted: "Horrific incident in our community."
"Our hearts are broken for the families of the four Marines killed in today's terrible act of violence. They are in our prayers," he added.
Eyewitnesses reported seeing the gunman firing from inside a car outside the recruitment centre.
Gina Mule, who works at a local restaurant, told CNN she heard "really loud noises" and saw a man with a "high-powered rifle".
"A lot of shots were fired," she added.
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".