A husband and wife traveling from Mexico to see their daughter in Washington were among the four identified Wednesday as those killed when a bus slammed into a highway sign that tore through the vehicle in California's Central Valley.
The identities were released as investigators started piecing together what led to the crash early Tuesday. Among the possible factors being examined are driver fatigue and mechanical problems.
"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.
Driver fatigue is a natural place to look considering the collision happened around 3:30 a.m., said Henry Jasny, senior vice president of Advocates for Highway and Auto Safety.
"We're daytime creatures," Jasny said. "Our bodies want to sleep at night."
After 10 hours continuously behind the wheel, bus drivers must be off for eight hours. Truckers are allowed to drive 11 hours, but have longer required rest periods at 10 hours.
It's unclear how long driver Mario David Vasquez, 57 of the Los Angeles area, had been behind the wheel. Investigators have not been able to interview him because of his injuries, California Highway Patrol Officer Moises Onsurez said.
Federal rules for bus drivers need to change, Jasny said.
"The consequences are very serious in terms of the numbers of lives at stake on a bus," he said.
There were 27 people, including the driver, aboard when the bus struck an exit sign post amid San Joaquin Valley farmland, officials said. Of the survivors, seven suffered major injuries, and 16 had minor to moderate 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 an adjacent county.
According to Capt. Brian Hawkins of the Merced CHP, the crash site was "chaotic" and "there was a lot going on." Offering condolences to the victims' families and friends, he said he was "glad" the death toll had been modified.
"I wish it was zero," Hawkins added.
The crash split the bus down the middle. The local sheriff has said rescuers pulled "bags of body parts" from the bus. Some survivors suffered severed limbs.
Investigators, meanwhile, are working to find out why the bus suddenly veered sharply off the highway, smashing into a guard rail and then a pole that tore through the vehicle.
Probes into such crashes can last months before a probable cause is determined, officials said.
The NTSB will also look into possible distractions or medical issues the driver suffered, as well as other drivers or hazards in the road and whether the bus company had a culture of safety problems. Their findings will also inform safety recommendations that officials hope will help to prevent "future tragedies," Karol said.
Officials said they want to give answers to the survivors and the relatives of those killed as well as make bus travel safe.
The dead include Fernando Ramirez, 57, and Petra Carillo Ruiz, 64, a married couple from Mexico traveling to visit a daughter in Pasco, Washington.
Also identified was Jaime De Los Santos, 38, of Tijuana, Mexico. Relatives describe him as a devoted family man with four children.
The final victim was identified as Jose Morales Bravo, 58, who lived most recently in Avalon, California, but came from Concepcion de Buenos Aires in the state of Jalisco, according to relatives. Officials said Bravo's wife was among those hospitalized.
Eva Maria Morales wept as she talked about her father, Jose Morales Bravo.
"He was a good father," she said in Spanish from her home on Santa Catalina Island, off the Los Angeles coast. "He loved being with his children."
She said her parents made sure to visit all their children at least once a year and had most recently left Catalina for Washington state, where two of them live. Morales said her mother is expected to survive.
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.
"It kind of ran him off the road," Nakia Coleman told Washington state's Tri-City Herald newspaper. Hawkins did not respond to Coleman's report, saying at a news conference that he didn't want to "speculate."
The bus was traveling north on State Route 99 when it crashed, only a few miles from its next stop, where it was scheduled to change drivers.
Passenger Leonardo Sanchez was sleeping peacefully on the bus carrying him to Oregon to pick blueberries when he was suddenly thrown face-first into the back of the seat in front of him.
He said only about eight people, including himself, escaped largely unscathed.
"We couldn't pull people out because there was shattered glass everywhere, seats destroyed," Sanchez said.
Some victims were thrown from the bus into a ditch.
Autobuses Coordinados USA was operating the 1998 Van Hool bus. Calls to various telephone listings for the company went unanswered.
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.
Separately, Karol said that the bus, being an older model, may or may not have an electronic control module, as it called, which is able to and did record data prior to the crash.
"It's up in the air," he said.
NBC Bay Area's Rhea Mahbubani contributed to this report.