According to the King County Sheriff's Office, the bus collided with an SUV near the corner of SW 136th Street and Ambaum Boulevard South in Burien shortly before 8 p.m. The bus careened off the road and ran into a vacant building.
At least three people were injured, the sheriff's office said, and the bus driver was trapped under a pile of debris when rescue workers arrived.
The three injured people were taken to area hospitals. No one else was on the bus, police said.
A woman and a boy in the SUV were spotted running from the scene, and police are hoping to question them about the crash.
The casino express is an independently run bus used to transport people to and from the Snoqualmie Casino.
The King County Sheriff's Office is investigating the cause of the crash.