Steve Neavling

The FBI is helping investigating a cache of explosives found under a bridge in Orange County Monday, the LA Times reports.

People working on a drain ditch found a backpack full of grenades, blasting caps and fuse igniters.

If ignited, the explosive would have caused “major structural damage to the bridge,” Orange County Sheriff’s spokesman Jim Amormino told the LA Times.

“It was a dangerous situation,” he said.

It’s unclear whether the explosives were rigged to detonate.