The Wisconsin State Patrol cited inattentive driving in a crash that killed a Milwaukee County sheriff's deputy this summer.
But the report said it's not known why Deputy Sergio Aleman did not react to traffic that had slowed to a stop in front of him.
Aleman was killed when the utility truck he was driving slammed into the rear of a flatbed tow truck on Interstate 43 on July 31.
The Milwaukee Journal Sentinel reported Aleman was traveling at 63 mph in a 50 mph zone. After braking, his vehicle struck the tow truck at 48 mph.
The report says a blood sample taken from Aleman showed no signs of illicit drugs or alcohol.
Adelman was 49. He had been with the sheriff's department for 18 years.