Three passengers were killed when the bus, carrying at least 35 people, plummeted 50 feet off a busy Seattle bridge toward an apartment building.
The victims included the driver Mark McLaughlin, 44, who lost control of the bus after being hit by gunfire. Mr McLaughlin died at the scene from multiple gunshot wounds to the abdomen.
Authorities did not reveal the gunman's name but said he may have been a passenger who shot himself in the head after firing at Mr McLaughlin.
Sources in Seattle identified the man as Silas Cool, a 43-year-old unemployed man.
The bus was ripped apart by the fall from the bridge.