The Vatican has for the first time opened its own investigation into the case of Emanuela Orlandi, a 15-year-old Vatican citizen who disappeared in the summer of 1983.
The Orlandi family's lawyer Laura Sgro confirmed the probe.
She said "the secretariat of state has authorised the opening of an investigation into a grave in the Teutonic Cemetery inside the Vatican," after an anonymous tip-off indicated that investigators should look where a statue of an angel in the cemetery is pointing.
The Vatican had previously said it was handling a request from the Orlandi family to reopen a grave close to the statue of an angel holding a sheet bearing the words Rest in Peace.
Interim Vatican spokesman Alessandro Gisotti declined further comment.
Emanuela was the daughter of a Vatican employee who lived with his family in the tiny city state.
She was 15 when she disappeared after a music lesson in Rome.
The family have long demanded to see Vatican documentation about the enduring mystery.
The cold case attracted fresh attention at the end of October when two sets of remains were found in the basement of the Vatican Nunciature, an extraterritorial Church property located in Rome's city centre.
The identification of at least one of the bodies as female led to immediate speculation in Italy that the findings might eventually shed light on one of the country's most persistent mysteries.
Investigators said at the time that preliminary examinations of the bones indicated they belonged to a woman probably in her thirties.
But, despite the age of the bones, the Orlandi family lawyer said they would wait for DNA results.
One hypothesis for Emanuela's disappearance is that she was not killed immediately but instead held for years against her will.
Investigative sources, however, told Italian media in the following weeks that the bones were too old to be related to the Orlandi case.
For more than 35 years, Italian media have been obsessed with the fate of Emanuela.
Over the years, many rumours have swirled about what happened to her - including conspiracies tied to the Mafia and the plot to assassinate Pope John Paul II.