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The three men missing in a 2-mile mudslide that washed over a rural area of Colorado were surveying a previous slide when the second one hit, officials said Monday after surveying the area.

The three are a county employee, his son and another man. The vehicles they were driving — a pickup truck and 4-wheeler — have not been found in the slide, which is a half-mile wide and 200 feet deep in some areas, said Mesa County Sheriff Stan Hilkey.

The men were investigating an interruption in ranch irrigation caused by a smaller, previous slide, Hilkey said.

“Everybody on this mountain is praying for a miracle right now — hoping they're stranded and avoided it,” Hilkey said of the community of Collbran, which he described as “tight-knit.”

“We’re still hopeful and still doing everything we can to make sure there’s not anybody left on these mountains that we can reach,” but debris was still falling and personnel could not safely search on the ground, Hilkey said.

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The search is being hampered because only the lower third of the slide is stable. Even at the edges, the mud is 20 to 30 feet deep.

“There’s a gigantic heap of dirt that could cause a secondary slide,” he said. Helicopters and unmanned vehicles were flying overhead to scour for the men, visually and with heat sensors.

“It’s an understatement to say that it was massive.”

Officials in Snohomish County, Washington, which suffered a slide that left 43 dead more than two months ago, were offering “good tips” on how to move forward in the search and recovery efforts, Hilkey said.

But Hilkey doesn’t believe the slide will allow for officials to execute a “cleanup” stage.

“I think what you’re looking at is the new terrain of that valley,” he said. “It’s an understatement to say that it was massive.”

"The slide came down with so much force and velocity that it came to a hill and went up and over the hill and came back down," he added. "And it was not a small hill."

The slide was reported at about 8:15 p.m. ET Sunday, but the ridge that collapsed was probably sliding most of the day, according to the Mesa County Sheriff's Office.

The area is remote, and no structures were reported damaged, but debris approached three active natural gas wells, according to the West Slope Colorado Oil & Gas Association.

Workers manually shut down the wells on Sunday, in case the slide continued to spread, said David Ludlam, executive director of West Slope. The wells showed no signs of spills or releases, and “we are pleased with preliminary reports showing minimally affected oil and gas production equipment,” Ludlam said.

A witness who reported the slide to authorities described hearing a noise that sounded like a freight train, according to the sheriff's office. Hilkey believes the slide was started by runoff caused by excessive rainfall.