Scotland Yard detectives investigating the disappearance of Madeleine McCann in 2007 are analysing the mobile phone details of everyone who was in the Portuguese holiday resort from where she disappeared.
They are set to reveal new information about the hunt for the missing girl in a televised appeal in 10 days.
There are around three dozen British police officers working on what has been designated Operation Grange, but the officers leading the investigation say there has been increasing cooperation with the authorities in Portugal.
Six Portuguese police officers based in Faro have been appointed to liaise with officers in London.
The Metropolitan Police stresses they are "professional and committed" and were not involved in the original investigation, which remains closed.
Assistant Commissioners Mark Rowley and Detective Chief Inspector Andy Redwood are also sending formal International Letters Of Request to 30 other countries asking for assistance with their inquiry, reflecting the range of nationalities likely to have been in the town on May 3, 2007.
Detectives want to cross reference mobile phone data with other lines of inquiry, especially with individuals they have previously identified as "persons of interest".
Madeleine was three years old when she vanished from her parents' rented apartment in the Algarve seaside town of Praia da Luz.
The Portuguese authorities closed their investigation after initially investigating her parents Kate and Gerry as possible suspects.
The Home Office agreed to order a review of the case by the Metropolitan Police and in July it became a full-blown investigation.
Officers have been looking into the backgrounds of 41 individuals, 15 of whom are UK nationals.
Assistant Commissioner Rowley said three of these British citizens were on the verge of being eliminated from the inquiry.
It is a huge undertaking requiring extensive international co-operation.
The phone data has always been available, and some of it has been examined before, but the trawl through thousands of phone numbers is the most thorough yet undertaken.
DCI Redwood said the phone records could be the key to solving the mystery and he insisted it was not a "general trawl" for information, but a "targeted attack" on possible key numbers.
"We've got a data set of phone traffic. Within that phone traffic you can see we've got some of those numbers we can attribute to people, but a large number of them we can't. So in a targeted way, we're trying to say in a particular moment in time, that is around the moment of opportunity, who's there.
"What we're trying to do is to use every route available to us to identify as many of them as possible and the phone data is one route into that, as are appeals. If you were in Praia da Luz at the time, you may get a routine phone call from the police."
The Metropolitan Police team has collated 39,148 documents from previous inquiries by both the Portuguese authorities and eight different teams of private detectives hired by the McCanns.
So far 21,614 have been processed. Some 4,920 of those have necessitated follow-up action and 2,123 lines of enquiry have been completed.
The new lines of enquiry will be publicised in a BBC Crimewatch programme on Monday, October 14.
The police will not confirm if new photo-fits or artist impressions of possible suspects will be issued, but there will be a reconstruction of events in Praia da Luz, and Madeleine's parents will be interviewed during the programme.
Mr Rowley said: "It's important to stress, the Crimewatch appeal is not simply 'this is a live investigation has anyone got any information?' It is more than that. There is new information not previously presented. Fresh, substantive material upon which to make an appeal."
The McCanns will be accompanied by DCI Redwood, making it the first time the couple have made an appeal for information alongside an investigating officer.
Every other public appeal they have made during the past six years has been on their own initiative.
Press conferences in the days immediately after their daughter's disappearance were not organised by friends and advisers rather than the Portuguese Judicial Police.
It will be a symbolic moment in the long inquiry , with Kate and Gerry McCann, once labelled "aguidos" in Portugal, and investigated as possible suspects, now officially supported by detectives investigating the case.
DCI Redwood said: "I have no reason to be anything other than confident in the McCanns. They have been thoroughly supportive of our inquiry and our relationship is very strong."
There will be similar appeals on TV programmes in Germany and the Netherlands, reflecting the high number of tourists from those countries who were in Praia da Luz when Madeleine went missing.