Rothschild Police Chief William Schremp said today one witness said it was raining very hard at the time the ambulance crashed Thursday night.
Fifty-two-year-old David Nicewander of Mosinee was being transported to the Wausau hospital by three EMTs.
The driver of the ambulance lost control of the vehicle on Interstate 39 and it flipped over.
Nicewander was pronounced dead at the scene. Two EMTs suffered serious injuries and the third was treated and released from the hospital.
It's not yet known if Nicewander died as a result of the crash or another medical condition.
Mosinee's fire chief declined to say why Nicewander was being transported.