BREA ( — Undelivered mail went up in flames early Tuesday morning after a United States Postal Service truck caught fire in Orange County.

The fire, which fully engulfed the big rig, started just after 1:30 a.m. near North Lamber Road on the 57 Freeway in Brea, the California Highway Patrol said.

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Witnesses said the truck appeared to “explode” as it was traveling down the freeway.

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It took firefighters about an hour to get the fire under control.

No injuries were reported and it was unclear how the fire was sparked.

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The big rig was not involved in a collision.