A helicopter unit and ground crews resumed the search Monday for two missing hikers in Cleveland National Forest.

The man and woman -- identified as Nicholas Cendoya, 19, and Kyndall Jack, 18, of Costa Mesa -- were reported missing after calling for help at about 8:30 p.m. Sunday. Their cell phone battery died, and the two friends have not been heard from since the call.

Authorities were unable to obtain an accurate location using GPS from the phone.

The search is focused on the Trabuco Canyon area. Dark and foggy conditions halted Sunday night's search, but skies cleared late Monday morning.

Rescuers used K-9s to follow the scent of one of the hikers more than a mile from where the pair had parked. Authorities said that somewhere near a mine shaft, the trail went cold.

"I'm having a feeling that one of them is injured and they are sticking together and they're just unable to be heard or seen," Orange County Sheriff's Department Lt. Erin Giudice said.

The hikers' vehicle was located at the entrance to Holy Jim Trail. Inside the vehicle were two phone chargers and a parking pass bought specifically for Sunday.

The search was expected to continue into Monday night.

