Russian police have stepped up the search for an American student who went missing on a hike in the mountains on Sunday.

Twenty-five-year-old Colin Madsen of Jefferson City, Missouri, disappeared a day after he arrived as part of a group from the east Siberian city of Irkutsk, where he was a university student. Madsen is believed to have ventured out before dawn in below-freezing conditions from a guest house in a mountainous stretch of Buryatia in eastern Siberia. The group planned to hike in the mountains later that day.

Police in Buryatia said in a statement Thursday that they were stepping up the search and that regional police chief Maj.-Gen. Oleg Kudinov is supervising it.

Madsen's parents were heading Wednesday to Russia to be closer to the search.