Updated: 10/11/10 6:34 p.m.

VAIL, Colo. (CBS) – Rescue crews have ended their third day of searching for a Chicago hiker missing in the Colorado mountains, with no sign of him yet.

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James Nelson, 31, was on a five-day, 25-mile hike on the Mount of the Holy Cross near Vail. Nelson is an experienced hiker and belongs to a backpacking club in Chicago, but this was his first trip to this area of Colorado.

The mountain 15 miles southwest of Vail is one of the best known of Colorado’s peaks over 14,000 feet.

Stan Bush from CBS TV’s KCNC in Denver reports that Nelson has the proper gear, but only enough food for five days.

Nelson hasn’t been seen since Oct. 3. His fiancee reported him missing Friday, after he was supposed to meet up with friends and never showed up.

Nelson is a records director for the Chicago-based Evangelical Lutheran Church in America and is known as a friendly, detail-oriented employee, according to John Brooks, a spokesman for the organization.

“A lot of people who work for this organization and who know him … are deeply concerned about him,” Brooks told CBS 2’s Kristyn Hartman in Chicago. “We certainly hope it all turns out well.”

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They’re not the only ones. There are messages about Nelson on Facebook and at the Chicago Backpackers Meetup Group website. Nelson has taught classes at a Lincoln Park sporting goods store.

Crews spent the last two days searching for him on the ground and in the air, covering a 25-mile loop. More than 70 rescuers are involved, including at least two military helicopters. Monday’s search was focused on areas off of the trailhead.

Nelson did not have a cell phone with him. He went out completely alone without any knowledge of the terrain.

Authorities have no idea what may have happened.

“He may have taken a wrong turn. He may have slipped and hurt himself. We’re looking at every theory right now. But we’re still looking for him. He could be hunkered down and he may be injured and somehow he just can’t communicate with us,” Eagle County Sheriff Joseph Hoy told CBS TV station KCNC in Denver.

Weather is going to be a big factor in the search. The temperatures get well below freezing at night. It snowed three inches above 10,000 feet over the weekend. When those conditions happen, it can be very hard to survive in the wilderness.

Rescuers came back empty-handed Monday. The search will resume Tuesday morning.

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