“It is concrete evidence, facts that we have, not mere indications,” Wolters said, adding that he was unable to disclose exact information.
“We have no forensic evidence of Madeleine's death, such as a corpse,” he added.
Police raised hopes in early June that the mystery over the disappearance of three-year-old “Maddie” could finally be solved when they revealed they are investigating a 43-year-old over her disappearance from the Portuguese holiday resort of Praia da Luz in 2007.
The suspect, who was not named by police but identified as Christian B. by German media, has a history of previous sex offences including child sex offences and rape.
Prosecutors had said they were working on the assumption that Madeleine is dead, though authorities in Britain had continued to treat her disappearance as a missing persons case.
Wolters said German authorities have written to Madeleine's family, but declined to give details of the content of the letter. He also confirmed that Christian B. has applied to be released early on probation from prison in Kiel, where he has currently served two thirds of a sentence for drug trafficking.
However, Wolters said the suspect will not be freed from custody as there is a case pending against him over the rape of an elderly woman in Portugal.
A court in Brunswick had sentenced Christian B. to seven years in prison last December for the assault against the 72-year-old American tourist in 2005 — in the same seaside village of Praia da Luz where Maddie went missing.
But the sentence has not yet been finalised pending extradition technicalities.
During the trial for that case, two witnesses said they had found a gun, video cameras and several tapes at Christian B.'s home in Portugal, according to a report in the Frankfurter Allgemeine Zeitung newspaper citing court documents.
One of them later watched some of the footage and saw a man raping an elderly woman tied to a bed, the report said.
Madeleine went missing from her family's holiday apartment on May 3, 2007, a few days before her fourth birthday, as her parents dined with friends at a nearby tapas bar.
According to police, Christian B. lived in the Algarve region of Portugal between 1995 and 2007.
He made a living doing odd jobs in the area where Madeleine was taken, and also burgled hotel rooms and holiday flats.
Police believe he was living in a white Westfalia camper van with yellow skirting at the time of the kidnapping and they are keen for witnesses who remember seeing the vehicle back then to come forward.
Christian B. has not been called in for questioning and his lawyers say he has so far refused to speak about the case.
The latest development in Maddie's case has prompted European investigators to reopen several cold cases of missing children.
In Germany, prosecutors are combing through the files to see if the suspect had anything to do with the disappearance of a five-year-old girl named Inga from the town of Schoenebeck in Saxony-Anhalt in 2015.
Authorities in Belgium said they have reopened a probe into the 1996 murder of a German teenager, Carola Titze, 16, in the resort town of De Haan on the Belgian coast.
And in Portugal, the stepfather of missing eight-year-old girl, Joana Cipriano, has asked authorities to explore possible links.