SAN FRANCISCO (AP) — Some Christmas cards and packages may not make it to their destinations after a U.S. Postal Service tractor-trailer caught fire near San Francisco, burning a lot of mail it was carrying.

KPIX-TV reports that the truck was heading from San Francisco to Los Angeles when the blaze broke out on Highway 238 Sunday morning.

Postal Service spokesman James Wigdel says officials are still determining the extent of the damage, but a “substantial amount” of mail burned. He says there was a mix of letters and packages inside the truck.

The California Highway Patrol says there were no injuries and the cause of the fire is still under investigation.