Updated 8:58 PM ET
RANCHO SANTA MARGARITA, Calif. Rescuers combing Southern California's Cleveland National Forest for two lost hikers said late Wednesday they weren't giving up hope the pair would be found alive despite a fruitless third day of searching.
There is no evidence of foul play and authorities believe the teens are in the area, in part because a 911 call made before their cell phone died was traced to a cell tower near the location, said Orange County Fire Authority Capt. John Muir.
"Their probability for survival is good," he said, adding that the nights have been mild and the days not too hot.
"We're not stopping until we find them."
So far, nothing has been found in the area where mountain bikers glimpsed what they believed to be a light in heavy brush Tuesday night off of a trail, he said.
The aerial search was to end at nightfall Wednesday but would resume Thursday at first light, Muir said.
Nicholas Cendoya, 19, and Kyndall Jack, 18, were last heard from Sunday night when they called for help on a cellphone. The phone's battery later died.
The two are believed to have gone off trail near Holy Jim Trail, a tree-lined dirt path along a creek that leads to a waterfall and is popular with day hikers.
In the call, they said they were about a mile from their car, which was parked at a trailhead, but rescuers have expanded the search.
"When you're disoriented because you're out of breath and tired and you think you're 1 mile away, you could be potentially 3 or 4 miles away," Muir said. "There's a lot of ground to cover."
It was unclear whether the lost hikers carried water and Kyndall's father, Russ Jack, said he worried that after three days the pair might be dehydrated. But he still has hope.
"So at this point everybody's still upbeat, optimistic about finding the kids in good shape and alive," he told TV reporters.
About 50 searchers, some on horseback and aided by dogs, moved back and forth through chest-high brush across mountain ridges.
The U.S. Forest Service gave permission to cut brush on a mountain peak to land a helicopter, which allowed LA County sheriff's personnel and two dogs to be taken to a canyon northwest of the pair's car, Orange County Sheriff's Department Lt. Erin Guidice told the Los Angeles Times.
That area was chosen because of the nearby cell phone tower that picked up the 911 call, she said.
Four helicopters also dropped search and rescue teams in the forest all day Wednesday, Muir said.
Jack's car was left in a parking area. Her mother drew a message on the dusty windshield that read: "Kyndall -- we r looking wont stop love you mom" and signed it with a heart. Jack's family towed away the car Tuesday night, KABC-TV reported.
The area is in a section of the national forest in the Santa Ana Mountains, which lie along the border of Orange and Riverside counties southeast of Los Angeles. The trail ranges in elevation from about 2,000 feet to about 4,000 feet.