Detectives investigating the disappearance of Madeleine McCann 10 years ago are still pursuing "critical lines of inquiry" and remain committed to providing her parents with answers, the officer in charge has said.
In an interview to mark the 10th anniversary since she went missing, Scotland Yard's Assistant Commissioner, Mark Rowley, said his team would never give up hope of finding out what happened to the youngster, the Daily Telegraph reports.
He said a number of critical lines of inquiry remained, including one lead that "could provide an answer", but said it was important to be realistic as the months turned into years.
Rowley said: "I so wish I could say we will definitely solve it but a small number of cases sadly don't get solved."
Madeleine was abducted from her parents' holiday apartment in the Portuguese resort of Praia da Luz on the evening of May 3, 2007.
The case sparked an unprecedented level of media interest but a decade later there have still been no definitive breakthroughs and her fate remains unknown.
For the past six years Scotland Yard detectives have painstakingly reviewed every piece of evidence in the case and have pursued all outstanding leads.
Rowley said although only four detectives were now working on the case full time, there was still useful work to do.
He said: "Where we are today is with a much smaller team focussed on a small number of remaining critical lines of inquiry that we think are significant.
"If we did not think they were significant we would not be carrying on, but we think they are significant."
He went on: "We don't have evidence telling us if Madeleine is alive or dead. It is a missing person's inquiry but as a team we are realistic about what we might be dealing with - especially as months turn to years."
He added: "Our mission here is to do everything reasonable to provide an answer for Kate and Gerry McCann.
"I'd love to guarantee that we will get to an answer, sadly investigations can never be 100 per cent successful, but we will do everything we can do reasonably to find an answer to what has happened to Madeleine."
He added: "We solve more than 90 per cent of serious cases here at Scotland Yard. I so wish I could say we will definitely solve it, but a small number of cases sadly don't get solved."
The initial Portuguese police investigation was closed in 2009, but two years later the British Government gave the Metropolitan Police funding to review the case.
As a result officers examined more than 40,000 documents and investigated about 600 people of interest.
In 2013, four suspects were identified and interviewed by police in Portugal, but all were eliminated from enquiries.
Police also undertook a dig close to the apartment from where Madeleine disappeared, but nothing of significance was found.
Rowley said the anniversary brought into sharp focus the pain and agony being endured by Kate and Gerry and the rest of their family.
He said: "As an investigation team we are only too aware of the significance of dates and anniversaries. Whatever the inquiry, we want to get answers for everyone involved.
"The disappearance of Madeleine McCann is no different in that respect but of course the circumstances and the huge public interest, make this a unique case for us as police officers to deal with.
"In a missing child inquiry every day is agony and an anniversary brings this into sharp focus. Our thoughts are with Madeleine's family at this time - as it is with any family in a missing person's inquiry - and that drives our commitment to do everything we can for her."