A Russian search is underway for a U.S. student officials say disappeared in freezing conditions after venturing from a guest house in Siberia.

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

Annie Madsen of Markle, Indiana, said Wednesday her nephew apparently left the house without a coat to take a walk in below-freezing temperatures while others in the guest house slept.

She says his mother from Missouri and his father from Indiana are heading to Russia.

The aunt says her nephew is fluent in Russian and has taught English there.