BERLIN — Russian heavyweight boxer Denis Boytsov has been placed in an artificial coma after being found with serious head injuries in a Berlin subway tunnel.

Boytsov, who has a 36-1 record, was found lying in the tunnel between two subway stations in the upmarket Charlottenburg district of western Berlin on May 3.

The 29-year-old fighter was placed in a medically induced coma because of swelling on the brain, a statement on Boytsov's website said Tuesday.

It was not until late Monday that Boytsov was publicly identified as the man injured in the incident. Although born in Russia, he has been based in Germany for more than a decade.

Berlin police said Tuesday that the case was still under investigation but that, based on witness interviews and video footage, the incident appears to have been an accident.

Police, who did not refer to Boytsov by name due to German privacy laws, said the man remains hospitalized but they had no details on his condition.

The statement on Boytsov's website suggested he had been the victim of an attack linked to his boxing career.

"It is possible that this attack was linked to the boxer's sporting activity," the statement said. "Around two years ago, Boytsov was receiving threats constantly."

There was no word on who had issued the alleged threats or why.

Boytsov last fought in March, when he beat Brazilian fighter Irineu Beato Costa Jr.

In 2013, he was the WBO's mandatory challenger to world champion Wladimir Klitschko, but lost that status after a defeat to Australia's Alex Leapai. Before turning professional, Boytsov was a world junior champion as an amateur.