Russian authorities are looking into the disappearance of Jefferson City man as a possible murder.
According to the Siberian Times friends of Colin Madsen were questioned using polygraph by police as the search goes on.
No trace has been found of Madsen, 25, who was studying at an Irkutsk university, after he vanished from a guest house in Arshan, Buryatia, at around 3 a.m. on Sunday.
While there is no hard evidence that he was killed, a murder probe has been opened, according to a statement from the Investigative Committee - seen as equivalent of the FBI - in the Republic of Buryatia.
"During the preliminary check, we have not confirmed that the missing young man was about to leave the resort village and his friends," according to the statement. "These and other established facts indicate that a crime may have been committed against him."
Madsen, a postgraduate student, had come to the thermal tourist resort in Buryatia with friends, including another American, from the Irkutsk State Linguistic University.
Three friends with whom Madsen was travelling are understood to have been questioned. Local TV channel Arig-Us reported the "first suspects" were his friends - understood to include one American - but the actual status of the questioning has not been disclosed.
The guest house was also checked for traces of blood. Madsen's friend Vlad Rasputin, 20, said: "They came yesterday, and checked the biological traces. But the forensics have no grounds, they have not found anything. ... They are interrogating all us with a polygraph."
Madsen's mother, Dana, arrived in Buryatia and is now in Arshan village with the investigators. The Emergencies' Ministry (EMERCOM) is using drones to search the area around Arshan, which includes the Peak of Love, a mountain that Madsen and his friends had been due to start climbing two hours after he disappeared.
"We plan to involve additional equipment - Phantom-3 unmanned aerial vehicles. With their help will be checked the forest, ravines, the bed of the Kyngarga River and other places, which are hard to reach."