The officer had left the keys in the ignition, Keller said.
When the police car's doors unlocked, Keller said the man jumped inside the cruiser and took off at a high rate of speed.
Nearby officers gave chase but were still several blocks behind when the man crashed about 3:10 a.m. into two vehicles at the intersection of 500 East and 3300 South, he said.
The cruiser plowed into the rear of the Mitsubishi Eclipse that was stopped at the light and then continued into the intersection where it struck a northbound Volkswagen, Keller said.
The driver of the Mitsubishi was transported to the hospital in extremely critical condition, Keller said. The Mitsubishi's female passenger remained hospitalized in serious condition Sunday afternoon. The driver of the Volkswagen was listed in serious condition, he said.
The man who stole the cop car was also in listed in serious condition, Keller said.
At minimum, Keller said police expected to arrest the man on suspicion of vehicle theft, stealing police firearms, and evading and resisting arrest. The man was expected to be arrested after he receives medical treatment.
Keller said Salt Lake City police were investigating the crash. The Utah Highway Patrol was investigating whether the man was intoxicated at the time, and South Salt Lake police are conducting their own internal investigation.
Keller said he didn't know why the man decided to steal the police car. He said the department was investigating whether the police officer, whose car was stolen, followed departmental procedures leading up to the incident.