"Gunman captured tonight by authorities," Washington State Patrol spokesman Sergeant Mark Francis said on Twitter yesterday. There were no other suspects.
The gunman opened fire with a rifle in the makeup section of a Macy's department store late Friday, killing four women and a man, according to police.
The FBI office in Seattle said it had no evidence the shooting was terror-related.
The shooter - described as a Hispanic man in his late teens or early 20s - later left the store on foot, triggering an intense manhunt.
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"I don't know what his motivation was to do this," Chris Cammock, police chief in the larger nearby town of Mount Vernon, told a news conference. "But I certainly plan to find out through the investigation, to the best of our ability."
It was the latest chapter in America's epidemic of gun violence.
Burlington Mayor Steve Sexton's voice trembled as he noted that the randomness of gun violence in America - which causes an estimated 30,000 deaths a year - had hit his small town.
The shooting came amid fierce debate in America over gun control laws. It is a hot issue on the campaign trail ahead of the November 8 presidential election.
The shooter had walked in without a weapon, and appeared on security cameras about 10 minutes later with a rifle, said Cammock.
Police received calls around 6:58 PM Friday that shots were fired at the mall.
The suspect was last seen walking toward a highway from the mall before officers arrived.
The mall was evacuated, police swarmed the area and medics rushed to the scene after the mall was initially placed on lockdown.
Police took hours to clear the sprawling building.