Russian authorities used helicopters Wednesday in the search for a missing American student who officials say disappeared in below-freezing conditions after venturing out before dawn from a guest house in a mountainous region of Siberia.

Colin Madsen, 25, of Jefferson County, Mo., disappeared early Sunday, investigators in the Russian republic of Buryatia said. Madsen arrived with another American student Saturday as part of a group from Irkutsk, where he was a university student.

According to region media outlets, Madsen vanished from the resort village of Arshan. The tiny town is known for its mineral springs and mountainous vistas.

The Investigative Committee, Russia’s federal investigative agency, said in a statement that Madsen and his group planned to hike the mountains Sunday, but Madsen “under unclear circumstance” vanished “without a trace” as others in the guest house slept.

The New York Daily News reported that Madsen left all of his belongings, including a cellphone, inside the house.

Annie Madsen of Markle, Indiana, said Wednesday 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 Wednesday to Russia to be closer to the search, his aunt said.

Madsen had been enrolled as a university student in Russia since the fall. His family said he was “inspired” by foreign cultures and the Russian language, according to the Daily News.

The village is located about 130 miles from Irkutsk.

The Associated Press contributed to this report.