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UK police have identified 38 "persons of interest" in the disappearance of Madeleine McCann in 2007.
The Met police said that they are moving the case from review to an "investigative stage of the inquiry" and will pursue a list of 38 "persons of interest" in the suspected abduction.
A police review opened in 2011 identified almost twice as many people whom detectives want to question in the case. A dozen of those identified are British nationals.
Madeleine's parents, Gerry and Kate McCann, said the shift from review to investigation was "a big step forward".
Police have spent the past two years digging through thousands of documents, pieces of evidence and leads to search for any fresh clues to what happened to the British toddler.
"Over the last two years what the review has told me is that there is no clear, definitive proof that Madeleine McCann is dead," Det Ch Insp Redwood, who is heading the investigation, named Operation Grange, told the BBC.
"So on that basis, I still genuinely believe that there is a possibility that she is alive."
A team of Scotland Yard detectives worked closely with Policia Judiciaria in Portugal before deciding to launch their own investigation.
The BBC's Danny Shaw said the investigation, assisted by police overseas, "offers the best opportunity to find out what happened to Madeleine McCann. It may also be the last chance to do so."
Madeleine disappeared on the evening of May 3, 2007while her parents were having dinner with friends at restaurant close to the Praia da Luz resort.
Despite thousands of tips and numerous sightings of Madeleine - who would now be aged 10- no sign of the little girl has never been found.