A car drove onto a closed road and struck two people in a crowd protesting against police brutality, killing one and critically injuring the other, authorities said.
Summer Taylor, 24, of Seattle died in the evening at Harbourview Medical Centre, spokeswoman Susan Gregg said.
Taylor and Diaz Love, 32, of Portland, Oregon, were hit by the car that barrelled through a panicked crowd of protesters on Interstate 5 early on Saturday morning, officials said.
Dawit Kelete of Seattle drove the car around vehicles that were blocking I-5 and sped into the crowd about 1:40am, according to a police report released by the Washington State Patrol.
Video taken at the scene by protesters showed people shouting “Car! Car!” before fleeing the road.
Ms Love is in serious condition in the intensive care unit, Harbourview, Ms Gregg said.
Ms Love was filming the protest in a nearly two-hour-long Facebook livestream captioned “Black Femme March takes I-5” when the video ended abruptly; with about 15 seconds left, shouts of “Car!” can be heard as the camera starts to shake before screeching tyres and the sound of impact are heard.
A graphic video posted on social media showed the white Jaguar racing toward a group of protesters who are standing behind several parked cars, set up for protection.
The car swerves around the other vehicles and slams into the two protesters, sending them flying into the air.
The driver, who was alone, fled the scene after hitting the protesters, Trooper Chase Van Cleave said.
One of the other protesters got in a car and chased the driver for about a mile.
He was able to stop him by pulling his car in front of the Jaguar, Mr Van Cleave said.
Troopers arrived, and the driver was put in custody, Washington State Patrol Captain Ron Mead said.
Kelete was described by offices as reserved and sullen when he was arrested, according to court documents.
He also asked if the pedestrians were OK, the documents say.
Kelete was booked into the King County Correctional Facility on two counts of vehicular assault.
Bail was denied.
A judge found probable cause to hold Kelete on an investigation of vehicular assault.
He faces a second court hearing on Monday at which the judge will determine if he can be released on bail, according to court documents.
It was not immediately clear if Kelete had an lawyer who could speak on his behalf.
Officials were trying to determine the motive as well as where he got onto the interstate, which had been closed by the state patrol for more than an hour before the protesters were hit.
Capt Mead said they suspect Kelete drove the wrong way on a ramp.
Trooper Rick Johnson said the driver went through a barrier that closed the road.
Troopers did not know whether it was a targeted attack, but impairment was not considered a factor, Capt Mead said.
Kelete has a Seattle address.
He is listed in public records as a student who attended Washington State University between 2011 and 2017 majoring in business and commerce.
His enrolment status could not be confirmed because the university was closed on Saturday.
The Washington State Patrol said on Sunday evening that going forward it will not allow protesters to enter I-5 and would arrest pedestrians on the road.
Seattle has been the site of prolonged unrest following the May 25 police killing of George Floyd in Minneapolis, which sparked nationwide protests.
Dozens of people were arrested this past week in connection with protests as demonstrations continue after authorities cleared the “Capitol Hill Occupied Protest” zone on Wednesday morning.
Protesters had shut down the interstate for 19 days in a row, Capt Mead said at a press conference.
The State Patrol responded by closing sections of the interstate to keep drivers and protesters safe.