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The California Highway Patrol released the recordings as investigators returned to the scene about 100 miles north of Sacramento to reconstruct aspects of the crash.
Dozens of injured students escaped through windows before the bus exploded into towering flames just before 6 p.m. April 10.
One student who escaped held back sobs in describing on a 911 call how the FedEx truck barreled across the median of Interstate 5 and smashed into the bus.
A dispatcher assured the student that medical help was coming and told the student to "go as far away as you can safely get" when he learned that the bus was still engulfed in flames.
Later, the dispatcher asked, "What did the bus hit?" and the student started to explain that the truck smashed into its left side. The dispatcher tried to refocus the student: "Just with one or two words, tell me what the bus hit."
"The bus hit a FedEx truck," the student replied. "The FedEx truck came into us."
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File - In this April 10, 2014, file photo, massive …
File - In this April 10, 2014, file photo, massive flames engulf a tractor-trailer and a tour bus ju …
"Was it head on?"
"Yes, head on."
It was not clear whether the student was a boy or girl. None of the 911 callers were identified.
Other calls came from witnesses and nearby residents.
"A bus just exploded," said one woman.
"It just exploded," said one man. "Whatever's on the freeway is on fire."
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The bus was carrying 44 high school students from the Los Angeles area for a visit to Humboldt State University on California's far north coast. Many stood to be the first in their family to attend college.
Five students and three adult chaperones died, along with the truck and bus drivers.
As the CHP released the recordings, the agency's investigators were reconstructing how the bus driver might have reacted to the sight of the big rig, which burst out of vegetation on the freeway's median into oncoming traffic, sideswiping a car before hitting the bus.
The CHP briefly closed the stretch of Interstate 5 where the crash happened and drove the same model Serta 2014 bus northbound at about 70 mph. The driver braked so investigators could gauge how its speed would have dropped.
On the southbound side, a driver in the same model 2007 Volvo truck released the accelerator, in a similar effort to understand how its speed might have changed.
Video cameras on both vehicles recorded what each driver could have seen before the crash.
The reconstruction did not include any collision. Investigators will use what they learned to calculate how fast each vehicle was traveling before the wreck.
The truck's data recorder was destroyed in the explosion and fire, but investigators said they may be able to recover some data about its speed and maneuvering by other forensic analysis.
Investigators are working through a 3-inch-thick stack of records including the truck's maintenance history and its driver's recent shifts, CHP Capt. Todd Morrison said. The FedEx driver had no prior moving violations, according to the California Department of Motor Vehicles.
It is too early to say whether mechanical failure or driver error caused the truck to careen out of control, Morrison said. That determination by the CHP, and by a parallel investigation by the National Transportation Safety Board, will take months.
The driver of the car struck first by the truck told investigators the truck was in flames before the crash, but Morrison said the CHP has found no evidence to corroborate that account. NTSB investigators also found no physical evidence of a pre-impact fire or other witnesses relating the same story.
Glenn County Coroner Larry Jones said all but two victims have been identified. He said that initially, his office was looking for one student believed to be among the dead, but it turned out the student had never boarded the bus in the first place.
911 calls capture chaos after truck-on-bus crash