Investigators from local and federal agencies have begun their probe into the fatal bus collision outside of Livingston on Tuesday, as the number of dead was changed from five to four.
The National Transportation Safety Board, along with the California Highway Patrol are investigating the crash and trying to determine what caused the bus to leave the freeway and plow into a large signage pole. The force of the impact caused the pole to shear through about half of the bus before coming to a stop.
The collision happened around 3:30 a.m. Tuesday on northbound Highway 99.
The CHP reported the bus was traveling at a high rate of speed when the driver lost control. The bus veered off the highway and into the Hammatt Avenue exit sign. The collision occurred just one stop away from where the bus was scheduled to exit and change drivers.
"At this point we don't know what occurred immediately before the crash," said Don Karol, a senior highway accident investigator for the National Transportation Safety Board.
Moments before the crash, it appeared the driver was trying to pass another vehicle that he apparently expected to yield, but it would not let him merge, passenger Nakia Coleman told Washington state's Tri-City Herald newspaper.
The investigation is expected to last several months and will look into possible distractions, sleep deprivation, or medical issues the driver may have experienced, as well as the involvement of any other drivers or hazards in the road and whether the bus company had a culture of safety problems.
The bus from Autobuses Coordinados USA had 27 people aboard when it crashed, including the bus driver, Mario David Vasquez, 57 of Los Angeles. The four dead were identified as: Fernando Ramirez, 57, and Petra Carillo Ruiz, 64, a married couple from Mexico traveling to visit a daughter in Pasco, Washington; Jaime De Los Santos, 38, of Tijuana, Mexico; and Jose Morales Bravo, 58, of Avalon.
Seven people sustained major injuries, including five that were airlifted from the scene. Another 16 people suffered moderate to mild injuries.
The California Highway Patrol initially reported five deaths, but Merced County Sheriff Vern Warnke lowered the count to four after canvassing hospitals and the coroner's office in Stanislaus County.
The bus was traveling from Southern California to Sacramento and eventually Washington. The Associated Press reported that the Federal Motor Carrier Safety Administration listed the carrier as having a "satisfactory" rating as of May 17. The bus was inspected in April and had three violations, including a lack of or a defective brake warning device.
The Associated Press contributed to the article.