Much of the search has been near Saddleback Peak and Holy Jim Canyon in the Cleveland National Forest.
Nicholas Cendoya, 19, and Kyndall Jack, 18, contacted authorities via cellphone around 8:30 p.m. Sunday asking for help.
They told deputies they believed they were about a mile from their car, but they couldn’t find their way back.
Authorities have located their car, a green BMW, but so far have found no sign of the hikers.
Their cellphone battery died before authorities were able to get an accurate GPS location for the pair.
“I believe they took a detour, I believe they did get off the trail and that’s where the trouble started,” said Orange County sheriff’s Lt. Erin Guidice.
“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.”
Authorities have been searching for the Costa Mesa hikers by air and ground, even using bloodhounds. They planned to have trail dogs and searchers on horseback on Tuesday.
Orange County officials are spearheading the search, with help from Los Angeles, Riverside and San Diego counties.
Anyone with information on the whereabouts of the teenagers is asked to call the Orange County Sheriff’s Department at 714-647-7000.
— Kellan Connor, KTLA