A gunman is at large after five people were killed in a shooting at a shopping centre in Washington state.

Police have released an image of the gunman, who they described as a Hispanic man wearing black and armed with a "hunting-type" rifle.

They say the motive for the shooting at the Cascade Mall in Burlington, 65 miles north of Seattle, is unclear.

Police have released an image of the shooter | Image via @wspd7pio on Twitter

The FBI said it is helping local authorities and is investigating the attack as a potential act of terrorism.

Its Seattle office wrote on Twitter that it "has no information to suggest additional attacks planned", but did not elaborate.

"We are still actively looking for the shooter," Washington State Patrol spokesman Sergeant Mark Francis said at a news conference.

"Stay indoors, stay secure."

Four women were killed and one man who suffered life-threatening injuries in the shooting on Friday evening died later in hospital.

All of the victims were found in the makeup section of a Macy's department store.

Tari Caswell told the Skagit Valley Herald she was in a dressing room when she heard "what sounded like four balloons popping".

She told the newspaper: "Then I heard seven or eight more, and I just stayed quiet in the dressing room because it just didn't feel right. And it got very quiet.

"And then I heard a lady yelling for help, and a man came and got me and another lady, and we ran out of the store."

"People were screaming"

A worker at a restaurant close to the shopping centre said staff "immediately locked the doors" when they realised what was happening.

Stephanie Bose, an assistant general manager at Johnny Carino's Italian restaurant, said a boyfriend of one of her employees came into the restaurant after trying to go to the shopping centre.

She said he saw people screaming and police blocking the door with assault rifles.

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

State governor Jay Inslee said he and his wife Trudi sent "condolences and prayers".

"Trudi and I send our condolences to the families of the victims and prayers for anyone injured. We urge residents to heed all safety and detour warnings.

"Stay close to your friends and loved ones as we await more information and, hopefully, news of the suspect's capture," he said.

The shooting comes less than a week after a man stabbed nine people at a shopping centre in central Minnesota before being shot dead.