Experts have discovered "thousands" of bones after opening up two ossuaries in a Vatican cemetery as part of an ongoing bid to solve the disappearance of a papal servant's daughter more than 35 years ago.

Emanuela Orlandi, a Vatican citizen whose father was a servant of John Paul II, vanished at the age of 15 in central Rome in 1983 in a case that has invited intrigue and conspiracy theories.

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Officials searched two 19th-century graves last week at the Vatican's Teutonic Cemetery, after an anonymous letter said to "look where the angel is pointing" - a presumed reference to an angel statue adorning the grave of Princess Sofia von Hohenlohe, who died in 1836.

However, the graves - that of Princess Sofia and Duchess Charlotte Frederica of Mecklenburg-Schwerin, who died four years later - were found to be completely empty.

In a bid to track down the remains of the noblewomen and potentially shed more light on the case, the two ossuaries were opened up.

"We did not expect to find so many bones. Today, thousands are being recovered, presumably from dozens of individuals," Giorgio Portera, who has been hired by the Orlandi family to look into the case, said, according to Italian media.

Now investigators are faced with yet more questions over the new remains discovered in the ossuaries. Portera spoke of "long, small and fragmented" bones and said it would take time to identify who they belong to.

The Vatican would only confirm that remains had been found and that an "in-depth morphological examination" is to follow next Saturday.

"This activity proves once again the dedication of the Holy See to the Orlandi family," Vatican spokesman Alessandro Gisotti said.