Washington State Patrol spokesman Keith Leary said in a phone interview that the suspect was taken into custody but declined to immediately release more details.
Seattle television station KOMO reported on Twitter that the suspect had been captured in Oak Harbor, a community about 50km southwest of Burlington where the mall shooting occurred on Friday (local time).
Washington state police officials said in messages on Twitter that authorities would hold a news conference to provide further details about the arrest.
The shooter disappeared under the cover of darkness at the Cascade Mall on Friday, authorities said.
The suspect, whose name has not been released, is believed to have entered the mall in Burlington, around 105km north of Seattle, and began shooting about 7pm (local time) in the cosmetics section of a Macy's department store, police said.
He initially walked into the shopping centre without the rifle but surveillance video later caught him brandishing the weapon, said Lieutenant Chris Cammock of the Mount Vernon Police Department at a briefing on Saturday.
