Story highlights Emanuela Orlandi vanished from Rome at the age of 15 nearly 30 years ago

A priest is suspected of complicity in her disappearance, a prosecutor says

Hundreds of demonstrators urge the Vatican to reveal what it knows about the case

Investigators open the tomb of a mob boss in search of the girl's remains

The brother of an Italian girl missing for nearly 30 years urged the Vatican to investigate her case as several hundred demonstrators carrying pictures of her marched to St. Peter's Square on Sunday.

The march came a day after Italian prosecutor Giancarlo Capaldo told CNN that a priest who used to run a church in Rome is under investigation on suspicion of complicity in the abduction of Emanuela Orlandi.

Msgr. Piero Vergari, the former rector of Sant'Apollinare, is being investigated along with four members of a criminal gang, Capaldo said Saturday.

Vergari, who left the position in 1991, did not reply to an e-mail from CNN seeking comment.

"We want the Vatican to open a serious investigation into the case," said Piero Orlandi, the missing girl's brother.

JUST WATCHED Mob boss' body exhumed from Vatican Replay More Videos ... MUST WATCH Mob boss' body exhumed from Vatican 02:38

Sunday's march is the latest attempt to get to the bottom of one of Rome's most enduring mysteries, the disappearance of Emanuela Orlandi, the daughter of a Vatican employee.

Orlandi, then aged 15, was last seen at her flute lesson at a music school on the grounds of Sant'Apollinaire late on the afternoon of June 22, 1983.

The case has spawned conspiracy theories pointing the finger at everything from the Mafia to the Vatican itself.

A top Vatican exorcist, the Rev. Gabriele Amorth, told CNN he suspected the girl had been abducted for sexual reasons, saying "The investigation should be carried out inside the Vatican and not outside."

Two weeks ago, police acting on an anonymous tip opened the crypt of a mob boss in the Church of Sant'Apollinaire in search of the girl's remains.

The Vatican had given investigators special permission to open the tomb of Enrico ''Renatino'' De Pedis following a 2005 phone call to a television program saying the truth would be found there.

Investigators found dozens metal boxes, all containing bones, inside the crypt that contained the remains of De Pedis.

Most are probably hundreds of years old but a source close to the investigation said some are much more recent.

Testing them will be "a long job, weeks or months," the source said, asking not to be named because he is not authorized to speak to the media. "It's like a puzzle of bones."

Supporters of the Orlandi family chanted "Truth, truth" in St. Peter's Square outside the Vatican on Sunday. Some wore headbands like the one Orlandi wore in one of the most reproduced pictures of her, while others held white balloons and banners.

Rome's Palazzo del Campidoglio displayed a giant poster of the missing girl on Sunday.

A former mayor of Rome expressed hope Sunday that the Vatican would do more to help with the investigation.

"The Vatican attitude has changed, they are more cooperative," said ex-mayor Walter Veltroni, who is close to the Orlandi family.

Veltroni made an appeal for anyone who knows the truth to come forward.

"I'm sure there are many people in Rome that know and that could help to discover the truth," he said. "It's time for them to talk."