MERCED, Calif. (AP) — On a warm August morning last year, charter bus driver Mario David Vasquez was drowsily talking on his cellphone as he weaved his white motor coach through Central California farmland shortly before barreling into a pole, killing four people, seriously wounding many more and nearly slicing the bus in half, prosecutors said Tuesday.

Vasquez’s extreme fatigue, violations of commercial regulations and repeated use of a cellphone while on the road led the Merced County District Attorney on Monday to file four felony counts of vehicular manslaughter and five misdemeanor vehicle code violations against Vasquez in connection with the Aug. 2, 2016 crash near the farming town of Atwater.

“Given their size and the potential dangers 18-wheelers and buses pose for other motorists, commercial drivers are appropriately held to the highest standards for safety under state law. Mr. Vasquez’s extreme fatigue, violations of commercial regulations and repeated use of a cell phone while entrusted with the care of passengers demonstrated a gross dereliction of the duty he owed not just to his passengers, but to every motorist on Highway 99 that morning,” said Larry D. Morse II in a statement Tuesday.

The wreck happened when the bus heading on a state highway to Washington state struck a large sign post head-on. Driver fatigue was believed to be a major factor in the crash, Morse said.

The post sliced through the middle of the bus, crushing rows of seats. Besides the passengers who died, several passengers who survived had to have limbs amputated.

A California Highway Patrol investigation found Vasquez was using his cellphone frequently while driving the bus carrying 26 passengers through California, including a call he was on a few minutes before the crash happened.

His commercial driver’s log book that he was required fill out showed he slept 6.5 hours the previous day, but cellphone records suggested he did not sleep that much, said Morse. Vazquez also violated laws linked to the maximum driving time allowed for commercial drivers, Morse said.

Vazquez has 30 years of bus driving experience. The misdemeanor violations stemmed from the driver’s alleged falsification of his daily log book, failure to keep accurate records and laws relating to maximum driving time for commercial drivers.

Surviving passengers described Vasquez as looking tired or drowsy during travel, and multiple drivers on Highway 99 in Merced County contacted investigators after the crash to report that the bus had been seen weaving shortly before the accident, Morse said.

Amanda McCoy, a spokeswoman for the Merced County District Attorney’s office, said she did not know if Vasquez has an attorney.WHAT OTHERS ARE CLICKING ON:

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