In January 1998 Fabio Tollis and Chiara Marino, both just 16, disappeared.

They had been drinking at a pub called the Midnight - the centre of the heavy metal scene in Milan - and they never came home.

Michele Tollis, Fabio's father, began to attend metal concerts and festivals across Europe, handing out leaflets and quizzing Fabio's friends.

Fabio and his friends were into the most extreme forms of heavy metal music - death metal and black metal, music obsessed with images of murder and satanism - and the role of this music is central to the story.

It emerged that Chiara, the girl who disappeared with Fabio, had a collection of satanic literature and paraphernalia in her bedroom.

During this search, Michele Tollis became convinced that satanism had something to do with his son's disappearance.

"No one can contradict me when I say that heavy metal and satanism are closely linked. They're inseparable," he says.

Over the next six years he built up a dossier of intelligence about them; who knew whom and which bands they played in.

Then in January 2004 Michele was watching the local news. An item came on reporting a brutal murder in their nearby town of Somma Lombardo.

A young man called Andrea Volpe had been arrested and admitted killing his ex-girlfriend.

Alarm bells went off in Michele's head because Volpe had played in a death metal band with his son. He called the police and arranged a meeting.

Using Michele's dossier of information, the police interrogated Volpe about the disappearances. Finally Volpe confessed and led the police to where Fabio and Chiara were buried.

Michele's hunt was over, but the revelations were just beginning.

One of Fabio's school friends, Mario Maccione, confessed to having beaten Fabio to death with a hammer.

He also revealed that the boys had been part of a wider satanic sect called the Beasts of Satan.

It was revealed Andrea Bontade, a drummer, had been terrorised into committing suicide.

Soon, other mysterious deaths were being linked to the Beasts.

The case has profoundly shocked Italy, still a very Catholic country.

One priest, Don Aldo Buonaito, has set up a helpline for parents and children worried about satanism.

He has also called for death metal music to be banned.

"If music makes itself an instrument of nefarious deeds and death it should be stopped," he says.

But this is just the latest case where extreme metal music has been blamed for vicious acts by teenagers.

Everyone remembers the secret messages that were supposed to be contained in Ozzy Osbourne's lyrics and the controversy over his track Suicide Solution.

In 1996, the parents of Elyse Marie Pahler took satanic death metal band Slayer to court in the US after their 15-year-old daughter was murdered in what was alleged to be a satanic ritual imitating the band's lyrics.

The case was dismissed by a Californian judge in 2001.

In Norway fans of the even more extreme black metal, burned down more than 40 churches in the 1990s.

One of the favourite bands of the Beasts of Satan sect in Italy was Deicide - an American death metal band led by Glen Benton, a self-professed satanist who has an upside down cross branded into his forehead.

Deicide sing about the occult, their most popular anthem being Kill the Christians.

Other death metal bands appear to glorify murder, torture, rape and necrophilia.

Benton is no stranger to controversy - several murders have been linked to his fans - but denies that any singer can be responsible for the actions of their fans.

"I say don't blame people like me and [Marilyn] Manson, because we never said: 'Hey, we're going to be role models for all your kids.' That ain't what this is about. It's about entertainment."

Indeed there is little evidence that ordinary kids can be turned into monsters by music.

But academics who have studied adolescents and music have expressed concerns about the possible effects on children who already have psychological problems.

Professor Don Roberts of Stanford University thinks that perhaps the children who are already violent or depressed are the people who should be kept away from death metal.

"What the music may well be doing is simply reinforcing beliefs that they might have started with in the first place," he says.

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