It happened just after 3 p.m. at the intersection of Highway 9 and Crown Ridge Boulevard, near Arlington High School.
The driver, an Arlington man, died at the scene. Police suspect he was intoxicated.
The people in the second car all were rushed to the hospital.
They were described by police as a 41-year-old woman, a 14-year-old girl and two toddlers, all from Marysville.
They all were in stable condition late Thursday night and were expected to survive, Arlington city spokeswoman Kristin Banfield said.
The dead man's name was not released Friday.
The Snohomish County Medical Examiner's Office completed an autopsy but needed to notify the man's family of his death before releasing additional details, a spokesman said Friday.
The man had been northbound on the highway in a red Mitsubishi when he crossed the center line and struck a black Nissan Leaf.
At the time, police already were en route to pull the man over, Banfield said.
Multiple drivers had called 911 to report the Mitsubishi "crossing the center line and fog line frequently," Banfield said.
One woman who was on the phone with 911 was a short distance ahead of the man on the highway before the crash.
The woman dropped her speed to 40 mph, which slowed the suspect, Banfield said. The speed limit along that stretch of the highway is 55 mph.
The slower speeds at the time of impact may have prevented the crash from being "much worse," Banfield said.
It's against state law to talk or text on the phone while driving, but contacting 911 to report an emergency is an allowed exception.
Detectives are just starting their investigation and it may take some time for them to reach conclusions, Arlington Police Chief Nelson Beazley said Friday.
After the crash, witnesses rushed to provide aid to the victims, Banfield said.
Firefighters had to cut the woman and the girl out of the car.
The accident closed the highway for hours.
Rikki King: 425-339-3449; firstname.lastname@example.org
More Local News Headlines
Objection delays release of records in school shootings 2:10 p.m. ‘Unprecedented’ wind storm: 24K in county still without power 12:32 p.m. Cost overruns, location, parking needs put courthouse on hold Clarence Robertson volunteered for 56 years as a reserve deputy 5-story building in Mill Creek would include affordable housing Island County undersheriff resigning to care for his children Front Porch: Light a candle for a loved one School winners
Our new comment system is not supported in IE 7. Please upgrade your browser here.