People fled, customers hid in dressing rooms and employees locked the doors of nearby stores after gunshots rang out just after 7pm local time on Friday at the Cascade Mall.
A helicopter, search teams and dog units scoured the area for a man with a rifle.
“We are still actively looking for the shooter,” Washington State Patrol spokesman Sgt Mark Francis said at a news conference. “Stay indoors, stay secure,” he added.
Sgt Francis said police were seeking a Hispanic man dressed in black and armed with a ‘hunting-type” rifle last seen walking toward Interstate 5.
Authorities say the motive was unknown for the shooting in Burlington, about 60 miles north of Seattle.
The FBI was assisting local authorities as dozens of police officers searched for the suspect.
The FBI’s Seattle office said on Twitter that it “has no information to suggest additional attacks planned” in Washington state but did not elaborate.
Authorities initially reported four people were killed, then said three were dead, before revising the number again. Francis said the number fluctuated as authorities searched the shopping centre.
A man was also critically wounded. No other details about the victims were provided, including their ages.
Tari Caswell told the Skagit Valley Herald she was in the women’s changing rooms at Macy’s department store when she heard ‘what sounded like four balloons popping.’
“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,” Caswell told the newspaper.
Stephanie Bose, an assistant general manager at Johnny Carino’s Italian restaurant near Macy’s, 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.”
She said he could see police at the doors with assault rifles and said they were no longer guarding the doors as of 9.30pm.
Sgt Francis said at about 8.30pm that the mall had been evacuated and emergency medical personnel were cleared to enter.
Sgt Francis added that authorities were still doing a ‘final clear’ of the 434,000sq ft mall late on Friday night.
He said 11 search teams and two dog units were involved.
The car park was closed and emergency management officials told people they would be able to retrieve vehicles on Saturday, although the mall itself is scheduled to be closed for the day.
Governor Jay Inslee said tragedy had struck the state.
“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,” Inslee said in a statement.