When the gunman opened fire from the driver's seat of his car, those inside the Armed Forces Career Centre, a suburban shopfront on Lee Highway in Chattanooga, Tennessee, hit the floor and then fled through a rear door.
It was around 10.45am when up to 30 rounds punched through the windows before the man, believed to be 24-year-old Muhammad Youssef Abdulazeez, a naturalised US citizen born in Kuwait, took off in a silver convertible Ford Mustang, witnesses say.
All of those in the recruitment centre survived that first attack. A witness told CNN that after the gunman unleashed this first fusillade he drove off of his own accord.
Other reports say that by the time he smashed through the gates of a Navy Reserve Centre around 10 kilometres away he was being pursued by police.
During this attack four US Marines were killed and, it is understood, a police officer shot through the ankle. The gunman was shot dead at this site.
During a press conference local authorities later described the incident as an act of domestic terrorism, only to be contradicted by an FBI spokesman, who said the motive for the attack was not yet clear.
The attack comes after US military facilities were put on heightened alert for the July 4 national holiday.
It is understood that Mr Abdulazeez lived in Hixson, a neighbourhood not far from the attacks.
Two other people have been wounded in the incident..
Officials said law enforcement authorities were investigating whether the gunman was inspired by the so-called Islamic State or a similar terrorist group, Reuters has reported. Abdulazeez, who held a degree from University of Tennessee at Chattanooga, was arrested in April for driving while intoxicated, local media reported.
US Attorney-General Loretta Lynch has directed the FBI to lead the national security investigation of the shooting.
"The US Attorney's office and department prosecutors are also actively involved," Ms Lynch said. "In the days ahead, we intend to work with our partners in law enforcement and the intelligence community to ensure that the American people are protected and that justice is served."
The attack has already prompted calls for US servicemen and women to be armed when on duty domestically.
The attack follows the massacre of 12 people at the Washington Navy Yard in 2013 and two separate shooting incidents at Fort Hood in 2009 and 2014, in which 13 people and four people respectively were killed.
One witness, Gina Mule, told CNN the gunman pulled up to the recruitment centre and opened fire without leaving his car.
"I looked out of our window and I seen the guy in his car, a silver Mustang, drop top, a white guy and he had a high-powered rifle and was just firing shots into the Air Force, Navy and Marines office," she said.
"I don't even know how many shots he fired, but it was a lot … After he got done opening fire, he pulled out really quick."
Another witness, Bruce Gans, told the Times Free Press: "We had the back door of the store open when we heard 'Pop, pop, pop,' which sounded liked an AR-15 [assault rifle]. Within minutes, it seemed, cops were all over the place. We heard rumours that five Marines had been killed. We locked the doors."
A photo of the recruiting centre broadcast on cable TV news station CNN showed its entrance riddled with bullet holes.
Chattanooga lies along the Tennessee River is in the south-eastern section of the state, just north of the Georgia border. Just over 173,000 people live there, according to a 2013 estimate from the US Census Bureau.
A White House spokesman said US President Barack Obama had been told about the shooting.
"The President has been briefed by his national security staff on the Chattanooga shooting, and will continue to get updates as warranted," said spokesman Eric Schultz.
with Reuters
