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German investigators believe that new 'circumstantial evidence' proves that convicted rapist Christian B murdered the little girl when she went missing in Portugal in 2007.
German prosecutor Hans Christian Wolters told The Sun: "We have gathered new evidence that adds further to the case we are building against our main suspect.
"I'm afraid I cannot tell you what it is but it strengthens our work.
"We are still building the case and at some time in the future we shall share the details with the suspect and his lawyer - but now is not the time.
"I cannot disclose the type of evidence we have been given, it is not forensic I can tell you that but it is new circumstantial evidence which all adds to the working theory that he is the man responsible."
Madeleine was taken just a few days before her fourth birthday while her parents, Kate and Gerry, were eating at a nearby tapas bar. The family was on holiday in Praia da Luz, in the Algarve, with Madeleine and her twin siblings left in the apartment alone.
Mr Wolters explained that Christian B remains the main suspect, saying that German police have 'concrete evidence' that he is responsible.
He said: "We have always insisted that the man we identified as the main suspect is the man we believe committed the crime and we are not looking for anyone else.
"I am optimistic that we will solve this case."
Christian B is serving seven years in prison for raping a 72-year-old American woman in Praia da Luz in 2005.
And he added: "We have found no evidence that suggests he is not guilty of this crime and we have found a lot of evidence that suggests he is guilty but I am forbidden to go into these details."
The news comes as Madeleine's parents, Kate and Gerry McCann, marked what would be their daughter's 18th birthday with a poignant message:
Posted on the official Find Madeleine Campaign Facebook page, the message read: "Happy 18th birthday Madeleine.
"We love you and we're waiting for you and we're never going to give up."
The birthday message came a week after the 14th anniversary of her abduction.
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