However, NTSB officials said they are not yet ruling anything out.
This finding comes after the driver of a Nissan Altima that was sideswiped in the crash said she saw flames coming from the FedEx truck before it collided with the bus.
A preliminary report on the crash investigation is expected in about 30 days.
Meantime, Los Angeles-area students were reunited Sunday with their families, their happiness darkly contrasted with grief for the one bus of fellow students that was markedly missing from Union Station.
That bus was involved in Thursday's crash on the I-5 near Orland. Ten people, including five students, were killed. Dozens more were injured.
For those who returned safely, their family members were relived and overjoyed. One student told her family that she was supposed to be on the bus that crashed, but a chaperone moved her. That student's mother said she is beyond relieved that her daughter is OK.
"I feel like she's born again for me, like God gave me her again," said the woman, only identified as Ana.
Their return was also marked by Palm Sunday. Prayers for the students and their families went up at Cathedral of Our Lady of the Angels in downtown Los Angeles, and at other churches across Southern California.
Many of the students said they were not fully aware of just how bad the accident was until they arrived at Humboldt State University for a two-day campus tour.
The NTSB continues to probe the crash. NTSB officials on Sunday said a fire expert found no physical evidence of a fire on the FedEx tractor-trailer prior to the crash, addressing claims of a pre-impact fire. However, NTSB officials said they are not yet ruling anything out.
A preliminary report on the crash investigation is expected in about 30 days. Officials were reviewing manteinance records and the FedEx driver's medical history, experience and cell phone records for possible distracted driving.
Federal safety investigators said the FedEx driver did not apply the brakes and there were no skid marks where the truck crossed the freeway median. Officials said the bus driver did try to avoid the collision. There is evidence of skid marks from the bus, and that the driver tried to swerve to avoid the crash.
The fatalities in the crash have been identified as Arthur Arzola, 26, a Humboldt State University recruiter from Rancho Cucamonga; chaperones Michael Myvett and Mattison Haywood, who were engaged; Adrian Castro, an El Monte High School student; Marisa Serrato, a student at Norte Vista High School in Riverside; Ismael Jimenez and Denise Gomez, students of Animo Inglewood Charter High School; Jennifer Bonilla, a student at Dorsey High School in Los Angeles; and bus driver Talalelei Lealao-Taiao.
The driver of the FedEx truck is the only fatality in the crash has not yet been identified.
Support continues to grow for the victims' families. The Red Cross has set up a 24-hour hotline to for any of the families involved. The Red Cross is providing mental health and shelter services, and travel or transportation assistance.
The Red Cross hotline can be reached at 1 (800) 540-2000.
The LAUSD has also established a 24-hour help line for any concerned parents: (213) 241-3841. The help line will be in operation indefinitely. Parents can also call Humboldt State University at (707) 826 6327.
A fund was set up by the Los Angeles Unified School District, Los Angeles School Police Association and the Friends of Safe School USA to help the families of the bus crash. For more information, visit www.laspoa.com.
Another fund, set up by Los Angeles City Councilman Joe Buscaino and Sharefest, was created to support the families. Those interested in donating can visit www.sharefestinc.org/buscrashfund.