More than 130 pages of records released Tuesday show investigators also believe the driver was speeding and may have become distracted or fallen asleep, The Arizona Republic reports.
The bus, with 22 people aboard, was heading from central Mexico to Los Angeles March 5 when it rear-ended a pick-up truck, veered across several lanes into the median and plunged into a ditch on Interstate 10 south of Phoenix.
No citations have been issued, and results of the investigation will go to the Pinal County Attorney's Office for review, the newspaper said.
The records from the Arizona Department of Public Safety show the bus's Mexican driver, Alfonso Alvarado, changed his story in several different interviews. A drug expert concluded he wasn't impaired.
Passengers disagreed on whether he had fallen asleep at the wheel.
Alvarado said the pickup slowed suddenly, which the pickup driver denied.
Other drivers reported Alvarado was speeding the morning of the crash and some said he tailgated a tractor-trailer for several miles, the records show.
Witness Wayne Shaw of Casa Grande told officers the bus had almost run him off the road when it sped past, weaving in and out of traffic.