An experienced hiker has gone missing in Colorado after summiting a mountain over the weekend, and his family is worried he may be disoriented in the wilderness without his epilepsy medication.

Ian O'Brien, 28, failed to return after setting off on a solo hike around 2 p.m. in the Echo Basin area of the La Plata Mountains on Saturday, Montezuma County Search and Rescue said.  

"Ian has an extensive background in survival in the wilderness," his sister, Molly O’Brien, told FOX31 Denver. "That’s if anyone could survive in the wilderness, it would be him, like 100% by himself. But the added component of not having his medication means that he’s likely having multiple seizures and being postictal (a post-seizure state) while in the woods and could be in a lot of danger right now."

O'Brien, of Page, Arizona, had made it to the peak of Hesperus Mountain, located northeast of Durango, and shared a photo of himself on social media around 6:30 p.m., Montezuma County Sheriff Steven Nowlin told the Durango Herald. The search for O'Brien began late Saturday night after he failed to return to his camp.

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Ian O'Brien missing persons poster

Ian O'Brien, 28, was last heard from after he summitted Hesperus Mountain in the Echo Basin area of the La Plata Mountains on Saturday evening, officials said. (Montezuma County Search and Rescue )

O'Brien’s family said there was a reported sighting of a disoriented man who was walking through the woods without a backpack, which would have contained his medication, GPS and other equipment.

searchers in mountains

Teams have been searching from Mt. Hesperus west to the Echo Basin, West Mancos River, and Transfer campground, though there has been no sign of the missing hiker. (Montezuma County Search and Rescue )

Nowlin, however, told the Arizona Republic that the sighting was likely not of O'Brien because the man was wearing different clothing.

A dozen search teams scoured the area since Tuesday morning, and there was still no sign of the missing hiker as of Wednesday afternoon, the sheriff said.

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The search area extends from Mt. Hesperus west to the Echo Basin, West Mancos River, and Transfer campground. In addition to ground teams, a U.S. Air Force Blackhawk helicopter from Kirtland Air Force Base searched by air. 

USAF Blackhawk helicopter

A U.S. Air Force Blackhawk helicopter helped search by air on Tuesday. (Montezuma County Search and Rescue )

However, with no sign of O'Brien and resources being shifted to help with nearby wildfires, officials were unsure how much longer the search would last.

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"I don't know how long the search will go after (Wednesday)," Nowlin told the newspaper. "It just depends."