Troopers said a black 10-gallon bucket, which was rolling around in the left westbound lane, caused the crash just before 4 p.m. near the bridge's midspan.

They said the driver of a sedan slammed on his brakes to avoid the bucket and caused a chain-reaction crash involving four other cars and a semi truck. Four of the drivers were trapped in their cars and had to be cut out of their vehicles by firefighters.

Six people were taken to Harborview Medical Center with non-life threatening injuries.

The semi was hauling approximately 20 gallons of paint at the time of the incident, and much of it was spilled on the roadway. Between 5 and 10 gallons of the paint ran down the storm drain and into Lake Washington, troopers said.

Traffic was backed up across the bridge in both directions for three hours after the crash.

Investigators are looking for the car from which the black bucket originally fell. Anyone who may have seen a bucket fly off of a vehicle on the roadway around 3:30 p.m. Monday is urged to call troopers at (425) 401-7788.

Crews from the Mercer Island Fire Department and Seattle Fire Department responded to the crash.