A preliminary report from the National Transportation Safety Board about a fiery crash between a FedEx Freight tractor-trailer and a bus in California that killed 10 won’t be issued for at least about another month.

Investigators are expected to be on the scene for another five to 10 days near Orland, Calif., north of Sacramento. The crash happened late Thursday afternoon along Interstate 5, after the truck crossed the median, hitting the bus traveling in the opposite direction.

The board said on Sunday while it has not ruled out the possibility that the truck was on fire before it crossed the median like some eyewitnesses claim,  so far it has found no immediate evidence of a pre-impact fire. It did confirm it has found no evidence the tractor trailer tried to brake before it crossed the median at about a 10-degree angle. They found more than 150 feet of skid marks from the bus as it tried to avoid being struck.

Investigators have reportedly recovered the electronic control module from the bus, which could hold some clues as to how the crash happened. The one from the truck was too heavily damaged. There is other physical evidence from both vehicles they will examine.

The bus was carrying high-school students from Southern California to an event at a college in the northern part of the state. Five students were killed, along with three chaperones. The drivers of both the truck and bus were also killed. More than 30 others of the near 50 passengers on the bus were injured, some seriously.

A final report from the NTSB could take around a year to prepare.