The California Highway Patrol says it does not yet know what caused a FedEx tractor-trailer to veer across a grassy highway median and slam into a bus, killing 10 people.

 

Lt. Scott Fredrick said at a news conference Friday that investigators will try to determine whether the FedEx driver fell asleep, experienced mechanical failure or lost control because of a separate collision on the southbound side of the freeway. It also will probe roadway and weather conditions.

 

The investigation could take as long as six months.

 

The bus was carrying more than 40 high school students on their way to visit Humboldt State University when it was struck by the FedEx tractor-trailer Thursday.

 

The crash occurred on Interstate 5 in Orland, a city about 150 miles northeast of San Francisco.

Officials say the high school students were participating in a program that invites low-income or first-generation students to visit Humboldt State University. The program was scheduled to start today. (AP)

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