KANSAS CITY, Mo. » A Russian search using helicopters pressed on for a U.S. student today, three days after officials say he went missing in below-freezing conditions after venturing out before dawn from a guest house in a mountainous stretch of Siberia.

Investigators in the Russian republic of Buryatia say 25-year-old Colin Madsen of Jefferson City, Missouri, disappeared early Sunday, a day after he arrived with another American as part of a group from Irkutsk, where he was a university student.

Regional media outlets reported that Madsen vanished from the tiny resort village of Arshan, known for its mineral springs and mountainous vistas.

Madsen and the rest of his group planned to hike in the mountains Sunday but Madsen “under unclear circumstance” went missing “without a trace” as others in the guest house slept, according to a brief statement by the Investigative Committee, Russia’s federal investigative agency.

Annie Madsen of Markle, Indiana, said today that her nephew, who she said is fluent in Russian and has taught English in Russia, apparently left the house without a coat to take a walk.

“He has a lot of friends concerned about him,” Annie Madsen told The Associated Press about the nephew she called “a responsible young man.”

“I can’t see him just getting up in the middle of the night and going for a walk in freezing conditions without a coat,” she said. “I don’t know what was going on.”

Colin Madsen’s mother from Missouri and his father from Indiana were heading today to Russia to be closer to the search, his aunt said.