The crash was reported about 2:30 p.m. in the 1000 block of Cedar Avenue, Marysville police Cmdr. Robb Lamoureux said.
A small sport utility vehicle was southbound when it was crossed into the northbound lane, he said. The SUV sideswiped a car and then hit a pickup head-on.
The SUV's driver, a 41-year-old Everett woman, was flown to a local hospital in a medical helicopter. Her injuries were described as life-threatening.
The pickup driver also was taken to a local hospital and later released, according to police.
The city's Collision Investigation Team will "try to determine what caused the SUV to drift into the oncoming lane of travel," Lamoureux said.
No arrests have been made.
Police did not respond to requests for information about the crash until late Wednesday afternoon.
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