German prosecutors say they may have to drop the investigation into a convicted paedophile suspected of killing Madeleine McCann if they do not receive more information from the public.
Investigators told the BBC they have substantial evidence that Madeleine is dead - but this is not enough to take the suspect to court.
The three-year-old disappeared while on holiday in Portugal in 2007.
The suspect, 43, has been named by German media as Christian B.
Hans Christian Wolters, a prosecutor in the northern city of Braunschweig - where detectives are leading the investigation - told the BBC: "We have evidence against the accused which leads us to believe that he really killed Madeleine but this evidence is not strong enough at the moment to take him to court."
The evidence is "strong enough to say that the girl is dead and strong enough to accuse a specific individual of murder - that strong," he said.
However he added: "One has to be honest and remain open to the possibility that our investigation could end without a charge, that it ends like the others have.
"We are optimistic it will be different for us but for that we need more information."
The suspect, a German man, is currently serving a jail term in Kiel, Schleswig-Holstein, for drug-dealing, having been extradited from Portugal in July 2017.
He is believed to have been in the Praia da Luz area in 2007, when Madeleine went missing. She had been there on a family holiday with her parents and siblings.
In December 2019, the man was sentenced to seven years for raping a 72-year-old American woman in the same Portuguese resort in 2005.
The rape conviction is currently under review in the German courts, according to German n-tv news.
German media say Christian B has also been investigated over the disappearance of a five-year-old German girl, named only as Inga. She went missing from a family party in Saxony-Anhalt on 2 May 2015 and has never been found.
Police say the suspect was regularly living in the Algarve in Portugal between 1995 and 2007 and had jobs in the area, including in catering, but also committed burglaries in hotels and dealt drugs.
A senior judicial source in Portugal told the BBC the joint investigation into the new suspect began after a tip-off in Germany in 2017.
Investigators were told by a friend of Christian B that the suspect had made a "disturbing comment" in a German bar, as they were watching news coverage of the 10th anniversary of Madeleine's disappearance, according to the source.
German prosecutors have previously said they are assuming Madeleine is dead.
However, the Metropolitan Police, who are working with their German and Portuguese counterparts, said the case remained a "missing persons" investigation in the UK because there is no "definitive evidence" as to whether Madeleine is alive or not.
Madeleine went missing shortly before her fourth birthday from an apartment in Praia da Luz, while her parents, Kate and Gerry, were with friends at a nearby tapas bar.
Her disappearance sparked a huge and costly police hunt across much of Europe - the most recent Met Police investigation, which began in 2011, has cost more than £11m.
The Madeleine McCann case: a timeline
- 3 May 2007: Alarm is raised after Madeleine is found to be missing
- September 2007: Kate and Gerry McCann are made "arguidos" - formal suspects - in their daughter's disappearance
- July 2008: Portuguese police halt their investigation and lift the "arguido" status of the McCanns and another man, Robert Murat
- May 2011: Prime Minister David Cameron asks the Metropolitan Police to help investigate. A two-year review follows
- March 2012: Portuguese police launch a review of the original investigation
- July 2013: Scotland Yard says it has "new evidence and new witnesses" in the case and opens a formal investigation into Madeleine's disappearance
- October 2013: Detectives in Portugal reopen the investigation, citing "new lines of inquiry"
- January 2014: British detectives fly to Portugal amid claims they are planning to make arrests
- December 2014: Detectives question 11 people who it was thought may have information on the case
- September 2015: The British government discloses that the investigation has cost more than £10m
- February 2017: Portugal's Supreme Court dismisses a long-running libel case against Goncalo Amaral, former head of the local police investigation, ruling that his book, which alleged the McCanns disposed of Madeleine's body, is protected by freedom of expression laws
- April 2017: The only four official suspects investigated by police are ruled out of the investigation but senior officers say they are pursuing a "significant line of inquiry"
- November 2018: An extra £150,000 is granted to continue the investigation. It is the latest in a series of six-month extensions which take the cost of Operation Grange to an estimated £11.75m
- March 2019: Netflix screens an eight-part documentary about Madeleine's disappearance. Her parents, who did not participate in the film, feel it could "potentially hinder" the police investigation
- June 2019: The UK government says it will fund the Met Police inquiry, which began in 2011, until March 2020
- June 2020: Police reveal that a 43-year-old German prisoner has been identified as a suspect in Madeleine's disappearance