VATICAN CITY (Reuters) - The Vatican said on Saturday it had opened an investigation into  reports that a former teenage altar boy, who allegedly repeatedly forced a dormitory mate to have sex with him, went on to become a priest. The allegations concerning the St. Pius X Institute, known as a pre-seminary, were made in a recent book and in Italian television reports. The pre-seminary is a residence inside the Vatican for altar boys who serve at masses in St. Peter s Basilica mostly presided over by priests, bishops and cardinals. At times they also participate in papal liturgies.  The boys go to Italian schools while they live in the Vatican, a sovereign city-state surrounded by Rome. The reports said that despite complaints to superiors against the teenager, he was accepted into what is known as a major seminary when he became an adult. He was later ordained a priest and is now serving in a parish in northern Italy, they said. The statement said an initial investigation in 2013 had resulted in  no adequate confirmation . A new investigation had been opened following the latest reports in order to try to shed  full light on what really happened , it added. 