Five people were taken to a hospital, including a 25-year-old man who was unconscious and in critical condition when he was cut out of the wreckage, the Seattle Fire Department said.
About 30 people were evaluated for bumps, scrapes and pains, but didn't require an emergency trip to the hospital, spokesman Kyle Moore said.
At least one driver and likely more will be cited, said Washington State Patrol Trooper George Englebright.
The cars were crunched about 6:30 a.m. on the freeway near Boeing Field where traffic often slows suddenly because of congestion.
"This area will go from 60 to 40 in a second and people aren't looking far enough ahead," he said. "That's typically what happens."
The mishap began with a five-car pileup on the left side of the northbound lanes. It was quickly followed by a four-car collision on the right side by drivers trying to avoid it. A three-car pileup followed in the backup, Englebright said.
"On top of this whole mess there's another five-car smash-up on the other side" of the freeway, he said, probably a case of drivers distracted by the aid cars and lights on the other side. That crash about 7:30 a.m. blocked only one lane, but slowed southbound cars.
Freeway traffic was moving again by 8:30 a.m. in both directions. At the worst point, northbound lanes were backed up 14 miles to Federal Way, said state Transportation Department spokesman Mike Allende.
At one time the fire department believed there were 20 injuries and called in five private ambulances to help city aid cars, Moore said. "It was a fluid scene."
The number of pileups at the same spot was unusual but they were typical of chain-reaction collisions in heavy traffic, Englebright said.
"Some people avoided it; some people didn't," he said.
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