Koistinen's car was forced into the westbound line of traffic and collided with a school bus carrying students.
Koistinen died at the scene of the crash. Two passengers were transported to local hospitals with serious, but non-life threatening injuries. All were wearing seatbelts.
The students suffered minor injuries.
Holmquest was cited for careless driving. She was not injured. The Argus Leader reports she is a Sioux Falls police officer. She remains active while officials investigate.