Dave Edgar, who worked for three years on the investigation, recently revealed his belief that the tot is still being held against her will in Algarve, blithely unaware of the global search.
According to The Sunday Express, Edgar claimed there is "every possibility" that Maddie, who would be 13 by this point, is still alive.
"When you get up beyond the main strip of the Algarve there's countless isolated properties where Madeleine could be being held," he told the publication.
"Obviously we had a number of lines of inquiry and some of those lines did lead us to properties which we followed up and handed over to the authorities."
Edgar also shared his belief that someone out there knows what happened to Maddie, and he's still on the hunt for any information.
"I would suggest that 10 years, while it seems a long time, would not feel like that to that person with information. What happened on that night would still be fresh in their mind," he said.
"It is not too late to come forward and tell the truth. We have seen in a number of cases recently where people have come forward and given evidence of events which took place many years ago."
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