Last Updated Apr 11, 2014 8:19 PM EDT
ORLAND, Calif. -- Investigators do not yet know what caused a FedEx tractor-trailer to veer across a grassy highway median and slam into a bus, killing 10 people, but they are looking into various factors, the California Highway Patrol said Friday.
Those factors include whether the FedEx driver fell asleep, experienced mechanical failure or lost control because of a separate collision on the southbound side of the freeway.
Federal investigators said Friday night they will review whether a stretch of California freeway where a bus carrying students was struck by a big rig should have had a barrier down the median to prevent head-on collisions.
In addition, the National Transportation Safety Board said Friday it will determine if a fire suppression system recommended but not mandated for buses would have made a difference in the crash that left 10 people dead.
The bus was carrying more than 40 high school students on their way to visit Humboldt State University when the FedEx tractor-trailer swerved across a grassy highway median and slammed into their bus in a fiery wreck Thursday.
Jeremy Lockett pulled over on the freeway and photographed the wreck.
"You could feel the heat from the flames like you were standing right next to it," Lockett told CBS News. "I've never seen anything like that."
A Los Angeles couple recently engaged in Paris was among the victims. Debra Loyd confirmed for The Associated Press on Friday the death of her grandson Michael Myvett and his fiancee, Mattison Haywood. The two were chaperones on the bus carrying high school students to Humboldt State University when it was hit by a FedEx tractor trailer Thursday night.
A spokeswoman for the Center for Autism & Other Disorders in Southern California says Myvett provided therapy for autistic children there for the past two years. Spokeswoman Stacey Price says Myvett proposed to Haywood in Paris in December.
Humboldt State University confirmed that Arthur Arzola, a staff member in the admissions office, was also among those who died in Thursday's crash. In a statement, the school said he had been accompanying the students on their trip.
"Arzola is remembered by colleagues at Humboldt State for his passionate commitment to helping low-income and first-generation students get into college," the school said. "He dedicated his career to that work."Meanwhile, a Southern California family is desperately trying to locate one of two identical twins who was on the bus.
Marisa Serrato, 17, has been missing since Thursday evening, said her brother Miguel Serrato, 23.
The other twin, Marisol Serrato, was on a different bus that wasn't involved in the crash and made it safely to the school Friday.
Serrato said a sheriff's deputy called his father at about 10:30 a.m. asking the family to collect Marisa's dental records because authorities believe she likely was killed.
"We're getting a little bit scared," Serrato said. He said his mother was booking a flight to head north.
Marisol was accepted to Humboldt, while Marisa was waitlisted there. The two Norte Vista High School seniors decided to check out the campus.
"My sisters are real religious, smart, A students," Miguel Serrato said. "They were going to get their high school diploma; they were looking toward the future."
Serrato said the Riverside family hasn't slept since learning about the crash on the news at their father's 65th birthday celebration the night before.
"We're trying to think positively," Miguel Serrato said. "I haven't gotten any sleep. I close my eyes and picture my little sister."
Marisa, or "Marisita" as the family calls her, was the baby of the family because she was born five minutes after her identical twin.
Norte Vista High Principal Susan Boyd said she's been in touch with Marisol Serrato since Thursday night, and counselors are at the school. Another Norte Vista student also was on the bus that crashed and is in stable condition at a hospital.
The school has not received any word on the missing twin, she said.
In Humboldt, Marisol has been trying to find her sister, calling around to hospitals for information, Miguel Serrato said.
"She says she believes that it's her fault because it was her idea to go check out that university," Serrato said. "She's devastated right now. I told her to keep her head up and have faith in God."
The crash occurred on Interstate 5 in Orland, a city about 150 miles northeast of San Francisco.
"Since these are such in-depth, detailed investigations, we don't expect to have a final report for a minimum of three months, 90 days," Fredrick said. "It could take as long as six months depending on what the investigation entails."
Someone kicked out a window on the bus, and many of those aboard squeezed through and ran for their lives to the other side of I-5 before the vehicle exploded in flames.
The 44 teenagers aboard, nearly half from the Los Angeles school district, were participating in a program that invites prospective low-income or first-generation college students to visit Humboldt State University in far Northern California.
Steven Clavijo, a high school senior from Santa Clarita who planned to enroll at the school, was trying to catch a nap on the bus when he felt the vehicle begin to shake from left to right and then he heard a loud boom.
"We knew we were in major trouble," he said.
After he escaped, two more explosions soon followed. Clavijo and other survivors looked on, knowing others were still trapped in the inferno.
Both drivers were killed, along with three adult chaperones and five teenage students, according to the California Highway Patrol.
Towering flames devoured both vehicles just after the crash, and clouds of smoke billowed into the sky until firefighters doused the fire, leaving behind scorched black hulks of metal. Bodies were draped in blankets inside the burned-out bus.
Three buses were traveling as a convoy and only one was involved in the crash, said Earl Perkins, assistant superintendent of operations for the Los Angeles schools district.
Nineteen of the students on the bus were from the Los Angeles schools, Perkins said.
The National Transportation Safety Board sent a team to investigate.
"Every piece of paper associated with this will be looked at," said Eric M. Weiss, an NTSB spokesman.
The bus was among three the university had chartered as part of its two-day Preview Plus program to bring prospective students from Southern California and the San Francisco Bay area to tour the Arcata campus, according to university officials. The other two buses made it to campus, and the university was providing those students with counseling.
In a statement, FedEx CEO Frederick W. Smith called it "a tragic day for these families, and our thoughts and prayers are with them." He also said FedEx is "committed to providing every resource necessary to assist investigators in their efforts to understand what happened."