Scores of volunteers, search dogs and sheriff's deputies on horseback were combing a state park Friday for a 12-year-old Georgia girl who disappeared Thursday evening.

Amber Nicole Swanson was visiting family in western Pennsylvania when she went with her brother and two friends, all in their 20s, to rugged and heavily wooded Laurel Mountain State Park.

The three men left the girl and went climbing around on some rocks, state police Sgt. Roger Pivirotto said. They said they called out to the girl, who responded, but that she was not there when they returned from climbing at about 7:15 p.m., Pivirotto said.

The men searched about an hour before notifying authorities, Pivirotto said.

"At this point, it's just a child missing in the woods and, of course, we're looking into any and all possibilities as to why this occurred," Pivirotto said.

State police used a helicopter to search the area overnight, and the Pennsylvania Department of Conservation and Natural Resources, which runs the state's parks, organized a search party that set out at dawn Friday.

Police have not said what part of Georgia the girl is from, and did not identify the men with her, except to say they are believed to be from Westmoreland County. The 493-acre park is about 45 miles southeast of Pittsburgh.

A Civil Air Patrol unit was being mustered to join the search and a Marine Corps reserve unit has offered a helicopter to aid in the search. The equestrian team from the Somerset County sheriff's office was also searching the rugged terrain on horseback.