Four women were shot dead and one man died of his wounds after being airlifted to hospital following the attack at the busy Cascade Mall in the city of Burlington.
The man, who was in critical condition, “did not make it through the night,” police spokeswoman Heather Axtman said.
Sergeant Mark Francis of the Washington State Patrol tweeted a picture of the alleged shooter, who was still at large, saying police believed there was just one gunman and that he was armed with a rifle.
The suspect apparently left the scene at Cascade Mall on Friday evening before the police arrived. He was described as “a Hispanic male wearing grey”.
Police did not comment on a possible motive behind the attack.
Francis said four of the victims had been found at Macy’s, a department store.
“We believe all five were [found at Macy’s] but it has not been confirmed if the fifth was yet,” he said.
A witness to the shooting, named Jo Zavala, said he “heard a lot of shots and people running out of Macy’s store.”
“Then I saw a man pointing a gun and shooting people. And then I got up to go look and he saw me, pointed the gun and I took off running,” he told local broadcaster K5.
When he returned to the store to look for his wife, Zavala found some of the victims, including two elderly women, lying bleeding on the floor, he said.
Francis said on Twitter that police were wrapping up a search of the shopping mall area and were “actively searching for [the] suspect.”