The gunfire erupted Saturday night when multiple shots were fired outside of the casino around 9 p.m.
Officers who responded found two men with gunshot wounds who were rushed to Harborview Hospital with serious injuries.
The suspected shooter was caught driving away from the scene and was taken into custody.
Police believe there was some sort of altercation among the men before the shooting erupted near the parking garage entrance.
They are not identifying the suspected shooter, but say he's being held on investigation for two counts of assault in the first degree.
As of Sunday, the victims were medically unable to speak with detectives, and police will not release their identities.
Much is left to be investigated, say detectives, and it is still unclear what exactly happened before the shooting.