U.S. Federal safety investigators say the driver of a Fedex tractor-trailer that struck a bus carrying high school students didn't appear to brake before a fiery collision that left 10 dead.

National Transportation Safety Board member Mark Rosekind said Saturday that the truck left no tire marks as it careened across a median and slammed into the bus taking the students to a college tour in Northern California.

Investigators also interviewed a witness who said the truck was already on fire before it crossed the median.

The truck and bus exploded into towering flames after Thursday's crash, making it difficult for investigators to determine whether a fire started in the truck. But Rosekind says investigators plan to look at a blood test and other physical evidence to make that determination. (AP)