Interstate 15 in Hesperia was closed in both directions Monday after a fire broke out at a bridge construction site.
The San Bernardino County Fire Department said workers were cutting reinforced steel bars with torches when the bridge's wood falsework ignited.
"The fire has completely engulfed the bridge. It's not looking good, and we can't do a determination of the damage," said Terri Kasinga, a Caltrans spokeswoman.
Although the freeway was initially closed because of fire debris, the blaze caused a "signficant collapse to the bridge," the San Bernardino County Fire Department tweeted.
Tim Watkins, a public information analyst with the San Bernardino Associated Governments, confirmed the report, adding that "right now we're seeing steel members that have come down to the roadway, actual bridgework that was used to support the freeway."
He said the freeway would be closed for at least another 12 hours.