By Steve Kingstone BBC correspondent in Sao Paulo

Francisco das Chagas has made a full confession

Thirty-nine-year-old Francisco das Chagas is a bicycle mechanic and a self-confessed serial killer.

He says that, starting in 1989, he murdered 41 boys and teenagers in the states of Maranhao and Para in northern Brazil. All of the victims were castrated.

For the last six months, Chagas has been supplying details to the police. He agreed to a BBC interview, which took place at a police station in Sao Luis, the state capital of Maranhao.

"I did this because something was guiding me, directing me," he explained. "It was like a voice in my head. And it was that thing - the voice - that determined what happened."

Grisly murders

For the interview, Chagas was wearing a green polo shirt, jeans and handcuffs.

He spoke softly but deliberately. I asked whether he was telling the truth about these crimes. Could he possibly be confused?

"I'm not confused, and I am telling the truth," came the reply, "and you're hearing it from me personally."

Bodies were found buried at the site of Chagas' former home

He is likely to face trial early next year. Since he has made a full confession to the police and a judge, what is at issue is not his innocence or guilt - but whether he understood his actions.

If a jury accepts Chagas' contention that he was not mentally in control of his actions, and that outside forces were guiding him, he will be confined to psychiatric care. If not, he will go to prison.

The police are sceptical about his explanation. Twenty minutes' drive from Sao Luis, Detective Joao Carlos Diniz shows me the rural plot where Chagas' home once stood. Officers found three bodies buried here - and the detective explains that Chagas volunteered his confession only after the grim discovery.

Chagas should have been caught after one or two murders. But for 30 boys to die here - it means the police didn't do their job Rita da Silva

"What happened here is what I call the 'chameleon effect'," says Diniz. "Chagas was clever - he blended into the scenery. And from the outside, it was almost impossible to spot him."

The detective's comments carry an implicit admission: that the police were slow in catching Chagas. In Maranhao alone, there were 30 murders before he was caught. Ten of the bodies were found in fields near Chagas' home.

Police negligence?

Failings by the police have prompted legal action by Brazilian human rights groups.

A complaint has been filed with the Inter-American Commission on Human Rights in Washington. It alleges that the Maranhao state authorities were negligent in investigating the killings.

Rita da Silva's son Jonathan was Chagas' last victim

"Chagas should have been caught after one or two murders," says Rita da Silva, whose 15-year-old son Jonathan was the last of the victims.

"But for 30 boys to die here - it means the police didn't do their job. Why? Because the victims were all poor."

Until now, the case has gone largely unreported in Brazil, but in the coming months, it will have far-reaching ramifications.

Already, five men are serving prison sentences for murders which Chagas now says he committed. Four of them are in the state of Para, where Chagas lived before moving to Maranhao.