Italian prosecutors have charged a 42-year-old man with killing a female student who suffocated during a sex-game gone wrong, media reported Sunday. Soter Mule tied Paola Caputo, 24, to another woman while performing a Japanese sado-masochist technique known as "shibari." The engineer was originally held for murder but authorities in Rome believe Caputo and her friend consented to the game. Investigators have so far established that the three spent the night drinking and using drugs at a club before Caputo, said to be Soter's long-term girlfriend, suggested going to the car park of the building in a Rome suburb where she worked as a caretaker. The two women remained clothed while they were tied tightly together with the same rope and suspended two metres off the ground. The balancing act meant that when the unnamed woman fainted, Caputo was suffocated. Mule tried to save her by cutting the cord before calling for help. He reportedly told investigators: "No one forced anyone. Paola and her friend consented. It's a horrible accident." Caputo's 23-year-old friend was taken to hospital in a serious, but not life-threatening condition. Nicknamed Kinbaku, Mule was known online for practicing bondage and was a member of the BDSM association, standing for bondage, discipline, sadism and masochism. "He was considered one of the experts," a person familiar with the scene told the Ansa news agency. "Practising this kind of extreme sex, everything has to be controlled, but he knew that completely. We don't understand what happened."