Scotland Yard detectives believe mobile phone records may hold the key to solving the Madeleine McCann case.
Three-year-old Madeleine McCann, of Rothley, Leicestershire, vanished on holiday in Praia de Luz, Portugal, in 2007.
Police are analyzing data from thousands of phones belonging to people in the village at the time. There are 41 potential suspects, they say.
A major appeal based on “substantive” new information will be broadcast on the BBC’s Crimewatch on October 14.
Madeleine McCann was days away from her fourth birthday when she disappeared from her family’s holiday apartment.
Detective Chief Inspector Andy Redwood, who is leading the inquiry, said officers were examining a “substantial amount of data” from mobile phones thought to belong to people who were in the resort of Praia de Luz in the days just before, during and after Madeleine McCann’s disappearance.
Police are trying to identify the owner of each phone to build up a picture of exactly who was in the area. More than 3,000 people live in Praia de Luz, while holidaymakers and seasonal workers visit from countries across the world.
“This is not just a general trawl,” said Det. Ch. Insp. Andy Redwood.
“It’s a targeted attack on that data to see if it assists us to find out what happened to Madeleine McCann at that time.”
Detective Chief Inspector Redwood said officers had so far been unable to attribute a “large number” of mobile numbers and admitted that it was difficult to do so with phones bought six years ago on a pay-as-you-go basis.
The records also contain information on which phone numbers were dialed and when calls were made. It is thought some phone numbers might appear on police intelligence systems or be linked to criminals.
“We can see what the phone is doing, but we can’t see the text messages,” said the detective.
“It shows a timeline of the call data.”
According to Scotland Yard, the phone records had been “looked at” during the initial Portuguese police investigation but not in detail.
Asked by reporters if the information held the key to the investigation, Det. Ch. Insp. Andy Redwood replied: “It could do.”
He said there was no CCTV available – evidence which is often used to help solve missing persons inquiries in the UK.
Scotland Yard announced it was launching an investigation into Madeleine McCann’s disappearance in July – after spending two years reviewing the case, under the codename Operation Grange.
At that time, detectives said there were 38 “persons of interest” from five different countries – Portugal, the UK and three others that were not named.
Police said the number had now gone up to 41, of whom 15 were UK nationals.
However, detectives said work was “pretty now complete” on three of the Britons and they were likely to be struck off the list in the near future.
No one has been arrested.
Since July, police have formally requested the co-operation of the Portuguese authorities and a team of six senior detectives from Faro, in the Algarve, has begun working on the inquiry. Portuguese authorities dropped their investigation into her disappearance in 2008.
Metropolitan Police Assistant Commissioner Mark Rowley said it was a “good and professional” relationship and it was hoped that in future a small group of Scotland Yard detectives would be based in the Algarve to work with the Portuguese.
“It’s easier to do it alongside than at a distance,” he said.
Law enforcement agencies in 30 other countries – most of them in Europe – have also been asked for their assistance, principally to trace people thought to have been in Praia de Luz at the time.
Appeals for witnesses and information are also expected to air in Germany, the Netherlands and, possibly, the Republic of Ireland – the countries where most of the tourists in Praia de Luz came from.
The Crimewatch programme will feature a reconstruction and interviews with Kate and Gerry McCann, who, for the first time, will appear alongside detectives working on the investigation.
Police said the investigation was “gathering momentum”, though much work was still to be done.
Of 39,148 documents from the various police and private investigator inquiries detectives from Operation Grange have processed 21,614 of them.
The number of police tasks, known as “actions”, to be carried out by the new 37-strong investigative team numbers 4,920, of which 2,123 have been completed.
Andy Redwood said police were working backwards from the moment Madeleine McCann went missing to understand what happened to her.