Twelve vehicles were involved in the first - and biggest - crash at 6:30 a.m., which blocked all northbound lanes of the freeway for about an hour and a half.
One person, a 25-year-old man who was trapped in a vehicle down an embankment, suffered critical injuries. He was extricated with heavy rescue tools and was rushed to Harborview Medical Center, said Kyle Moore of the Seattle Fire Department.
In addition, five others were taken to area hospitals for treatment of less serious injuries.
About 30 people were evaluated for bumps, scrapes and pains, but didn't require an emergency trip to the hospital, Moore said.
Investigators say the crash was a chain reaction pile-up involving three separate collisions. It was started by a speeding motorist who was following too close, the State Patrol says.
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, Washington State Patrol Trooper George Englebright said.
It happened in an area 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," Englebright said. "That's typically what happens."
As emergency workers were clearing the first crash, another one happened about 7:30 a.m. in the southbound I-5 lanes in the same area. Five people sustained minor injuries in that crash, which involved five vehicles.
Englebright said the second crash was probably a case of drivers distracted by the aid cars and lights on the other side. That crash blocked only one lane, but slowed southbound cars.
Ricky Oliba, who was involved in the second crash, said he was hit by another driver.
"She hit me on the side of my car … so I barely made it," he said.
The freeway was a sea of flashing lights for more than an hour as multiple medical aid units responded to the crash scenes, along with fire trucks, Washington State Patrol troopers, police, sheriff's deputies and five private ambulances.
At the worst point, northbound lanes were backed up 14 miles to Federal Way, said state Transportation Department spokesman Mike Allende.
"It's been rough, rough - I'm late to work and everything," said Levi Robinson, who was stuck in the backup.
But he said he got off easy compared with the crash victims.
"While this is terrible, it's not as bad as the day they're having," he said.
The driver who started the chain-reaction crash and probably others will likely be charged, the State Patrol reported.
Freeway traffic was moving again by 8:30 a.m. in both directions.