MUNICH, Germany (Reuters) – A German sports doctor accused of masterminding an international blood doping network for professional sportspeople, from at least 23 athletes from eight countries over many years went on trial in Munich on Wednesday.

The case is a result of “Operation Bloodletting” under which police raided the Nordic Ski World Championships in Austria in February 2019 and arrested athletes just hours before the start of an event. In Germany, the doctor was arrested.

Prosecutors say the defendant, identified in Germany only as Mark S., was behind performance-enhancing blood transfusions mostly for cross-country skiers and cyclists.

They believe he was involved in the practice from at least the end of 2011.

There are two forms of blood doping. Autologous blood doping is the transfusion of an athlete’s own blood, which has been stored, refrigerated or frozen, until needed. Homologous blood doping is the transfusion of blood taken from another person with the same blood type.

“The charge is the commercial use of a banned doping substance and is directed against the doctor and the small group of people who are alleged to have helped him over a long period of time,” prosecution lawyer Anne Leiding told reporters.

Four other suspects are on trial accused of helping him with the collection and supply of blood.

If convicted, the doctor could be put behind bars for between one and 10 years. The trial is set to continue until at least mid-December.