The four mountain bikers who went missing in the Angeles National Forest on Sunday were not familiar with the trail they were on and were ill-equipped for the elements, authorities said.
The search began about 7 p.m. when the four cyclists were several hours late returning from their planned ride from Mt. Wilson to the Jet Propulsion Laboratory Windsor turnout in Pasadena, according to the Los Angeles County Sheriff's Department.
Sheriff's Deputy Dan Paige told NBC Los Angeles that it appeared it was first time the men — who range in age from 28 to 38 — had taken the trail.
He also said the group was ill-equipped to handle the elements overnight in the forest, where temperatures have been in the mid-40s.
"They just had camel packs and T-shirts," Paige said.
Search-and-rescue teams, aided by a helicopter, searched for the men until about 4 a.m. Monday, according to authorities.
The cyclists, who were not immediately identified, are from Glendale, North Hollywood, Rialto and Sunland. The Sheriff's Department said family members found automobiles belonging to the riders at the Windsor turnout.
Plans were being made to bring in additional resources for the search effort, which began anew around 7 a.m., according the Sheriff's Department.
"We plan on searching until we find these gentlemen," Paige told NBC.