The suspect entered the Cascade Mall in Burlington, around 65 miles (105 km) north of Seattle, and began shooting at about 7 p.m. local time on Friday in the cosmetics section of a Macy's department store, police said.
The unidentified suspect, who police described on Twitter as an Hispanic male, initially walked into the shopping center without the rifle but surveillance video later caught him brandishing the weapon, said Lt. Chris Cammock of the Mount Vernon Police Department at briefing on Saturday.
The rifle was later recovered at the mall, said Cammock, who is commander of the Skagit County Multi-Agency Response Team.
Four women were killed in the rampage, which police believe was carried out by a lone gunman. Later a man who was seriously wounded in the shooting died at a local hospital. None of the victims were identified.
Steve Sexton, the mayor of Burlington, described the shooting as a "senseless act."
"It was the world knocking on our doorstep and it came to our little community here," he said before acknowledging the response by law enforcement. "I know now our support goes with them to bring this son of a bitch to justice."
Authorities offered no information about a possible motive for the attack, which followed a series of violent outbursts at shopping centers across the United States, including the stabbing of nine people at a Minnesota center last weekend.
"We have no indication that we have a terrorism act," said Michael Knutson, assistant special agent in charge of the Federal Bureau of Investigation's Seattle office. "I can't discount that, but I can't conclude it either."
After the shooting, police and rescue workers worked their way through the mall, clearing stores and evacuating shoppers, some of whom locked themselves in dressing rooms. The mall remained closed on Saturday as investigators sifted for evidence and attempted to recreate the crime scene.
Cammock said police had no clues about the identity or whereabouts of the suspect, and asked the public for help in tracking him down.
Authorities released a grainy photo of the suspect taken by a surveillance camera. It shows a young male in his late teens or mid-20s with short dark hair, dressed in dark shorts and T-shirt and carrying a rifle.
Local authorities searched through the night for the gunman and warned residents to remain indoors, though later said the area was safe.
The suspect was last seen walking toward an interstate highway that runs past the mall, which is 45 miles south of the Canadian province of British Columbia.
When asked why police had described the suspect as Hispanic, Cammock told reporters he believed those who saw the photo made the statement based on his dark complexion.
The shooting comes less than a week after a man stabbed nine people at a mall in the central Minnesota city of St. Cloud before being shot dead by an off-duty police officer. The FBI is investigating that attack as a potential act of terrorism.
Banner Photo Credit: Twitter @saksivas_