Witnesses said two motorcycles were speeding.
The freeway was blocked just south of State Route 520 at Mercer Street.
The fire after the crash was seen on Department of Transportation cameras at about 5:15 a.m. Tuesday.
Trooper Chris Webb said the motorcycle hit another vehicle and the rider was ejected. The motorcycle traveled 75 to 100 feet north of where the rider was ejected and then caught fire.
The rider, a woman, later died at the hospital. Police said the bike she was riding had been stolen out of Everett in June.
The Washington State Patrol said troopers are looking for a second motorcycle whose male rider is believed to have been traveling with the motorcyclist who died at the scene. The rider stopped, tried to put out the fire, and then continued northbound, according to witnesses.
Witnesses said the same rider slowed when they drove by the scene again and then continued on.
Witnesses said both riders were traveling at a very high rate of speed in excess of 100 mph. The crash scene is about 400 feet long.