British detectives say the key to the disappearance six years ago of Madeleine McCann could emerge from a "targeted attack" on mobile phone data from thousands of people at the Portuguese resort of Praia da Luz at the time the child went missing.
The line of investigation was disclosed as it was announced that Madeleine's parents, Gerry and Kate, would appear on the BBC's Crimewatch television programme. It will broadcast a reconstruction of the events before Madeleine, then three years old, vanished and the McCanns will appeal for anyone in the resort at the time who has yet to come forward to contact police.
Similar appeals will be broadcast in Germany, the Netherlands and Ireland.
The "footprints" from the phones of holidaymakers, staff and local people who were in the vicinity of the McCanns' holiday apartment in Praia da Luz when Madeleine vanished in May 2007 are being examined and cross-referenced with other information coming into the inquiry team.
Forty-one individuals from several countries, including the UK, are being investigated as persons of interest by London's Metropolitan Police.
Official requests for assistance have been sent to 31 countries, including Portugal, to identify other people whose mobile footprints have emerged in the sifting and resifting of tens of thousands of documents, communications data and eyewitness statements.
The new British investigation began as part of a £5 million (HK$63 million) review of all the evidence after an appeal for help by Madeleine's parents to Prime Minister David Cameron.
It is not known how many of the 41 - among them 15 Britons - are considered suspects.
Police will analyse phone data to track any calls that might have been made from the resort to others outside it, as they investigate whether there was any kind of ring involved in the abduction.