The 32-year-old paedophile was caught after the body of four-year-old Bosnian refugee Mohammed Januzi was found in his car boot.
The little boy had been snatched as his parents visited a welfare office on 1st October - and Silvio S’s mother, suspicious of her son’s behaviour, tipped off the police.
After they questioned Silvio, he confessed to taking the boy home and sexually abusing him.
He said that he strangled the little boy the next day ‘because he wouldn’t stop crying’.
During the interview, Silvio S also confessed to murdering six-year-old boy Elias, who was taken from his home in July.
Police spokesman Stefan Redlich said: “The man confessed overnight that he had also killed Elias.”
It is believed that the little boy - whose body was found buried in Silvio’s garden - was also sexually abused and strangled.
Detectives in Germany now want to know if the 32-year-old double killer took five-year-old Inga Gehricke - known in the press as the German Madeleine.
The blonde-haired, blue-eyed girl vanished in May this year after walking into woods during a family barbecue in the Saxony-Anhalt state, some 50 miles from Berlin.
And Scotland Yard officers investigating the disappearance of Madeleine McCann - who vanished from her family’s holiday apartment in Praia da Luz when she was three-years-old - also want to speak to Silvio S.
They are reportedly tracking the case and will be contacting their German counterparts to see if Silvio S was in Portugal in May 2007, around the time of Maddie’s abduction.
Kate and Gerry McCann have yet to publicly comment on these reports.
However a source has said that the pair have “every confidence” that Scotland Yard will investigate all possible leads.