Nearly 120,000 letters and bills went up in flames early Tuesday when a pair of big rigs collided along the 57 Freeway in Brea.
One was from a U.S. Postal Service facility in Santa Ana, where workers process about 1 million pieces of mail daily, according to officials.
The letters that were burned had originated from Orange County and parts of the San Gabriel Valley and were being trucked to Ontario Airport at the time of the crash.
The mail that caught fire had been marked first class, but because it was not certified, officials say they cannot track whose mail burned.
The letters would have been sent out of state or at least out of the Southern California area.
Authorities are investigating the accident, which happened around 1:30 a.m. Tuesday when the truck stuffed with the mail rear-ended another truck, leading to a long closure of the 57 Freeway near Lambert Road, according to the California Highway Patrol's traffic incident website.
Officials say anyone with letters that may have burned should check with the intended recipients over the coming days.
The Postal Service will issue documentation saying a fire caused mail to be lost. Customers can call (800) 275-8777 for more information.