(AP) — A severely sleep-deprived driver and a bus company with a poor safety record were causes of an August 2016 crash in California’s Central Valley that killed four passengers and injured 20 others, including the driver, federal safety investigators said Monday.

The National Transportation Safety Board said the driver had only slept about five hours over the 40 hours preceding the Aug. 2, 2016 crash. The bus traveling from Los Angeles to Modesto drifted off the right side of Route 99 and struck a highway signpost that nearly sliced the bus from nose to tail.

There were no signs the driver tried to stop or steer back on the highway.

The NTSB said a contributing cause was inadequate oversight by the federal agency regulating bus safety that allowed Fresno-based Autobuses Coordinados USA Inc. to continue operating despite a poor safety record. It called on the Federal Motor Carrier Safety Administration to change its rating system to make sure companies either fix serious safety issues or be taken off the road.

“Here’s yet another fatal crash involving both a motor coach carrier with a starkly evident history of safety problems and a severely fatigued driver,” said NTSB Chairman Robert L. Sumwalt. “It’s time that the Federal Motor Carrier Safety Administration move more aggressively to keep these unsafe carriers off American roadways.”

The company had failed eight of 29 federal inspections in less than two years and its out-of-service rate was nearly five times greater than the national average, yet it received a satisfactory rating from the FMCSA, the NTSB said.

Less than three months after the crash, the administration shut down the carrier after giving it an unsatisfactory safety rating, FMCSA spokesman Duane DeBruyne said Monday.

The incident was one of several fatal bus crashes in early morning hours involving fatigued drivers, including 2011 crashes that killed 15 in New York City and one that killed four in Doswell, Virginia, the NTSB said.

Fatigue was blamed in a crash that killed 13 people on Interstate 10 near Palm Springs on Oct. 23, 2016 when a charter bus traveling from a casino plowed into the rear of a big-rig whose driver had fallen asleep during a freeway closure. The truck driver was recently charged with 13 counts of vehicular manslaughter with gross negligence.

The NTSB said the August 2016 crash outside of Livingston would not have been as bad if a stronger guardrail prevented the bus from striking the signpost.

The board recommended guidelines to determine where stronger barriers should be installed to protect heavy vehicles, such as buses, from obstacles and hazards.