Prosecutors have insisted they “remain optimistic” they will crack the Madeleine McCann mystery – as the 14th anniversary of her disappearance approaches.
Hans Christian Wolters said a German investigation into suspect Christian Brueckner, 44, is running “at full speed”.
And he hoped a new TV documentary will help uncover more clues.
He told the Sunday Mirror: “Even if the public does not notice it much, our investigations in the Maddie case continue to run at full speed.
“We still assume the accused, Christian B, killed Madeleine. Even though the investigation of a case that will be 14 years old in a few days is very difficult after all this time, we remain optimistic and hopeful that we will be able to solve the crime.
“In doing so, we continue to work closely with the Metropolitan Police and the Portuguese law enforcement authorities, regardless of Brexit.”
Mr Wolters said in June last year that he believed Madeleine is dead and identified convicted paedophile Brueckner as a suspect.
Brueckner is serving seven years for the rape of a 72-year-old woman in Praia da Luz, where Madeleine vanished from an apartment on May 3, 2007 – days before her fourth birthday.
She was in Portugal with her parents Gerry and Kate and her twin siblings, then aged two.
A Danish-made documentary on Brueckner will screen in the suspect’s native Germany on April 28.
Mr Wolters added: “I did a long interview. Of course, we always hope for new leads. In this respect, the programme may well help to clear up the case.”
A lawyer for Brueckner has repeatedly called for police to show their evidence – and vowed his client will never face trial over Madeleine.
Parents Kate and Gerry, of Rothley, Leics, have said: “We will never give up hope of finding Madeleine alive.”