The Sound Transit bus traveled for half a mile after hitting the sport utility vehicle on an interstate overpass in Kirkland, KOMO-TV, Seattle, reported Tuesday.
A 76-year-old man in the SUV died at the scene, A 75-year-old woman passenger died later in a hospital, authorities said.
The driver of the SUV was hospitalized in critical condition.
One person in the bus was injured and taken to the hospital for treatment. About 35 people were aboard the bus at the time.
Washington State Patrol trooper Julie Judson said it was unclear what led to the crash and that the bus driver may have hit several other vehicles before finally stopping in the carpool lane of northbound I-405.
Judson said the bus may have some sort of mechanical problems. The bus driver reported the brakes failed and said he could not stop.