The Hispanic man, aged in his twenties, is armed with a rifle.

Police spokesman Sergeant Mark Francis said authorities believed there was only one shooter in Friday night’s attack at Cascade Mall in Burlington, describing him as a “Hispanic male wearing grey”.

Police released a closely cropped still from video surveillance of the young-looking suspect showing only his head and torso, though the full picture, published by local media, showed him holding what appears to be a rifle.

Four women were killed and one man who suffered life-threatening injuries later died in hospital, county officials confirmed early on Saturday morning.

Police received calls around 6:58pm Friday (0058 GMT Saturday) that shots were fired at the mall, located about 110 kilometres north of Seattle in the US Pacific Northwest state, Trooper Rick Johnson, another spokesman, told CNN.

“Right now, we can’t assume anything. So they’re still interviewing witnesses and trying to get a good idea of exactly what action to take next. We’re just asking people to, obviously, stay away from the area,” he said. “Stay inside and obviously report anything suspicious that they may see.”

The suspect was last seen walking toward a local highway from the mall before police arrived, Francis said on Twitter. The mall was evacuated, police swarmed the area and medics rushed to the scene after the mall was initially placed on lockdown.

‘SHOT AFTER SHOT AFTER SHOT’

Witnesses told KOMO News that a shooter walked into Macy’s department store and opened fire. Nearby businesses were evacuated, the television station added.

“I hear one shot and then stand kinda still and like two, three other people start running out saying ‘gun’ and then shot after shot after shot,” said Armando Patino, who was working at a cell phone store.

“I turn around and run to the store. Some people didn’t know where to go. I just moved them into the store,” he added. “We went in the back where we have a door and everything and we just stayed there until they told us to evacuate.”

Stephanie Bose, an assistant general manager at Johnny Carino’s Italian restaurant near the Macy’s store at the mall, said she immediately locked the doors to the restaurant after hearing about the shooting from an employee’s boyfriend.

“He was trying to go to the mall and people were screaming,” she said. “It was frantic.”

Brandi Montreuil said she was at the movies when they were told to leave. “I didn’t know anything,” she said. “The theatre attendant came in and apologised for stopping the movie and said they were asked to have everyone leave immediately.

“We didn’t know if it was a fire so we were looking for smoke of some kind. Then a few officers started canvassing through the crowds asking if anybody saw anything.”

She said an officer with a large gun started yelling for people to leave, and fast.

Multiple law enforcement agencies and sniffer dogs were searching for the suspect. Survivors were transported by bus to a nearby church to meet their families.

The FBI’s Seattle field office said it was assisting local authorities in reviewing intelligence and was providing additional manpower. “At this time, #FBI has no information to suggest additional attacks planned in WA state,” it said.

Police took hours to clear the sprawling building, with Francis explaining that there were “a lot” of locked rooms and doors. They are now focusing on the investigation.

Authorities were due to provide updates during a press conference at 8:00am (1500 GMT).

The mall said it would stay closed on Saturday as a mark of respect for the victims.

“We are deeply saddened by the tragic event that occurred this evening. Our prayers and condolences are with the victims and their families,” a Facebook message read.

State Governor Jay Inslee responded, tweeting: “Tragedy has struck in Washington tonight. Our hearts are in Burlington where a shooter has taken the lives of at least four people.”

Officers have secured the building, but several roads remain closed in the area as 11 search teams and two canine teams hunt the gunman.