Chagas confessed to killing Rita da Silva's son, Jonatham

A court spokeswoman said that, in his opening testimony, Francisco das Chagas Rodrigues Brito had confessed to the murder of a 15-year-old boy.

Mr Chagas reportedly told the judge he had killed 30 boys - not the 42 the police say he has confessed to.

The 41-year-old added he had been driven to acts of violence because he had been sexually abused as a child.

In a 2004 interview with the BBC, Mr Chagas said he had killed the boys because "... something was guiding me, directing me. It was like a voice in my head. And it was that thing - the voice - that determined what happened."

"Sometimes I'm revolted by what I did," he said, "but you must understand that something was using me to do this. Good people will understand that."

Sentence

Mr Chagas was arrested in April 2004 after police found the remains of two minors buried in his yard.

Chagas: 2004 interview

Prosecutors say they have charged him in connection with one killing - that of Jonatham Silva Vieira - because it is the one for which they have the most evidence.

Mr Chagas, a bicycle mechanic, told the court in the state of Maranhao that he had strangled Jonatham - but he denied hitting him, raping him, mutilating his body or using his blood.

Reports have suggested that all his victims were castrated.

If convicted, Mr Chagas faces a possible 30-year sentence. But his jail term could be reduced if the judge accepts the defence's argument that he suffers from psychological problems.