Nicholas Cendoya, 19, and Kyndall Jack, 18, called for help Sunday, saying they thought they were about a mile from their car but couldn't find their way back. Their cellphone battery died before police could get an accurate GPS reading.
Authorities have since located the pair's green BMW but the teens were not found.
Air and ground searches so far have proved fruitless, and authorities added mounted search units and trail dogs to their effort Tuesday, KTLA-TV, Los Angeles reported.
"The growing concern would be that they have injured themselves and they are under the tree cover and we're unable to find them by air," said Orange County sheriff's Lt. Erin Guidice.