Prosecutors have filed felony vehicular manslaughter charges against a California bus driver in connection with a wreck last year that killed four people and nearly sliced the bus in half.

The Merced County District Attorney’s office on Monday filed four felony counts of vehicular manslaughter and five misdemeanor vehicle code violations against driver Mario David Vasquez in connection with the August 2, 2016 bus crash near the central California farming town of Atwater.

The wreck happened when the bus heading to Washington state struck a large highway sign post head-on.

A California Highway Patrol investigation found Vasquez slept 6.5 hours the previous day.

Authorities say he also violated laws relating to maximum driving time for commercial drivers.