Image caption Madeleine was three when she disappeared from her family's holiday apartment in the Algarve in 2007

Mobile phone records may hold the key to solving the disappearance of Madeleine McCann, detectives believe.

Madeleine, of Rothley, Leicestershire, was three when she vanished on holiday in Praia da Luz, Algarve, in 2007.

Scotland Yard officers are analysing data from phones belonging to people in the village at the time - 41 people of interest have been identified.

A major appeal based on "substantive" new information will be broadcast on the BBC's Crimewatch on 14 October.

Madeleine 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 thousands of mobile phones thought to belong to people who were in the resort of Praia da Luz in the days just before, during and after Madeleine'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 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."

'Call timeline'

Det Ch Insp 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 dialled and when calls were made. It is thought some phone numbers might appear on police intelligence systems or be linked to criminals.

It's not just a bland 'Can you help us?' appeal. There is different material and a different understanding to be presented Mark Rowley, Metropolitan Police Assistant Commissioner

"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.

Jim Gamble, the former head of the UK's Child Exploitation and Online Protection Centre told the BBC's Today programme he had recommended the "cell dump" was looked at again in his 2010 review of the case.

Asked by reporters if the information held the key to the investigation, Det Ch Insp 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'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.

Data retention

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 Madeleine's disappearance in 2008.

What can phone data tell us? As mobile phones constantly send and receive data from mobile phone masts, a user's location can be identified to within a few hundred metres using triangulation techniques. Modern smartphones with GPS built in can be located far more accurately. Mobile phone records include the numbers of the call sender and receiver, the call duration and time. Couple this with location information and you can establish where and when callers made or received a call. This information is often used to verify or knock down alibis in criminal cases. The difficulty for investigators is establishing the identity of the user if the phone is pay-as-you-go (PAYG) rather than on a pay-monthly contract linked to a bank account. PAYG phones, SIM cards and top-up cards can be bought in-store for cash, leaving no identifying trail for investigators to follow. And because such phones can be lent or sold to other people, establishing exactly who made a telephone call is made even more difficult.

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 da Luz at the time.

Speaking to the BBC's Today programme, Peter Sommer, an expert on cyber security, said the "multi-jurisdictional nature" of the case, which would involve mobile phone companies in different countries - and the gap in time - could make it harder to track people down.

But he said "cell site data" was routinely used in most criminal court cases in the UK.

Mr Gamble said the EU data retention directive, which compels telephone companies to retain call and internet records for a period of time, was at an "immature stage" in 2007.

But he said it appeared the data "wasn't properly or appropriately interrogated," at the time.

In UK investigations, he would expect the data to have been examined almost immediately, he said, but the "complex nature and geography" had made it more difficult.

'Gathering momentum'

Detectives said "fresh and substantive" information would emerge on 14 October, when the BBC broadcasts a Crimewatch appeal.

Mr Rowley said: "It's not just a bland 'can you help us?' appeal. There is different material and a different understanding to be presented."

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 da Luz came from.

Image caption Police issued a computer-generated image (r) of how Madeleine might have looked aged nine

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.

Det Ch Insp Redwood said police were working backwards from the moment Madeleine went missing to understand what happened to her.

"It's like peeling back the layers from an onion," he said.