BOULDER, Colo. -- Boulder County sheriff's officials on Sunday called off the ground search for a 21-year-old Marine in Eldorado Canyon State Park.

Sheriff's officials said they would proceed with the case as a missing person investigation.

Lance Hering was last seen Tuesday, when he was hiking with friend Steve Powers. Powers told authorities that Hering fell and suffered a head injury that caused him to lose consciousness. Powers went for help, but his friend was not there when Powers returned with rescuers to the scene.

"We're pretty confident he's not in the search area," sheriff's Cmdr. Phil West said.

At times more than 100 people have searched for Hering. Despite the search being officially called off, a group of friends and family came together Monday to search for Hering.

"There's a guy up there working for the military and he says there's still hope," said searcher Chris Breithaupt. "If the family cares and this guy says go, we'll check it out."

A division chief with the sheriff's department said with his 32 years experience he believes it was the widest-range, longest-running search of its kind.

A pair of socks were found Sunday about one mile away from where Hering was last seen. Searchers said it is a long shot that they belong to him, however.

Authorities were considering the possibility that Hering, who returned a month ago from Iraq, reverted to a military mind-set and went for cover.

His family is concerned that Hering is disoriented and wandering. His father, Lloyd Hering, said his son suffered a memory lapse after a head injury when he was 11.

"I want to call for the people of Colorado to keep looking for this kid, because I honestly believe there's a good chance he's out there," Lloyd Hering said. "Every person in Colorado is a searcher for Lance Hering. He could be at a convenience store, a gas station. He doesn't drink so he probably wouldn't be at a bar. But he could be anywhere."

Hering is due back at Camp Pendleton in California in a couple weeks. Investigators said they don't believe he intentionally disappeared to avoid returning to Iraq.

Anyone who spots Hering is asked to call the Boulder Sheriff's Office immediately.