The Portuguese police's dramatic decision to focus on Kate and Gerry McCann was triggered by the results of crucial forensic tests sent from Britain this week.
The preliminary test results are understood to have found significant DNA evidence - including traces of Madeleine's blood - in the McCanns' apartment and car and this has caused detectives to target their investigation on the toddler's parents, particularly since the McCanns only hired the vehicle 25 days after the Madeleine's disappearance.
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Clues: new DNA evidence was found in a Renault Scenic hire car, like that shown above, and at the McCanns' holiday flat below
It is also believed that further forensic evidence obtained from other samples connected with the McCanns will be put to the couple during their separate interviews today.
Police are also focusing on the potential gaps and inconsistencies in the accounts given by the McCanns and their friends about the night Madeleine disappeared.
Until now, police have appeared satisfied to rely on the McCanns' account of events which suggests there was less than an hour between the last time Gerry saw the toddler and the time Kate discovered she was missing.
But with the McCanns now in the spotlight, one crucial question the Portuguese detectives want answered is when Madeleine was last seen by someone other than the McCanns.
Police sources in the Algarve suggest that initial concerns about discrepancies in the versions of events given by the McCanns and those who dined with them on 3 May - when Madeleine vanished - partly led to the decision to conduct forensic tests on the McCanns' hire car, an actthat would have been pointless if detectives had been fully convinced by the accounts of that evening.
As Kate McCann's questioning continued today after yesterday's 11-hour grilling - with detectives reportedly wanting to ask her 22 new questions - sources said police were fitting together a number of pieces in a complex 'jigsaw' of events, with some 'pieces' relating to this week's forensic results and others relating to information accumulated by the Portuguese police.
What is understood is that once these crucial elements were brought together a pattern of events and evidence emerged which raised increasing doubts about the McCanns.
"This is a very complex investigation," said one source. "It is like assembling all the pieces of a very big jigsaw.
"What is happening now is about both the evidence sent from Britain and the material the Portuguese have gathered themselves and how they fit together."
The key to the developments of the last 24 hours - which have stunned the McCanns' family and friends - lie in the test results sent to Portugal by experts from the Forensic Science Service laboratory in Birmingham.
A large volume of potential evidence was sent to the scientists around a month ago, including material from the McCanns' holiday apartment and hire car and from the nearby home of Robert Murat, the Briton previously
named as a suspect in the inquiry.
Material from a number of other unknown locations was also examined by the British scientists who were looking for traces of blood, saliva, hair and fibres that would shed light both on Madeleine's fate and those responsible for her disappearance.
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The Renault Scenic car was only hired 25 days after Madeleine's disappearanceFor the police, the purpose of these tests was to establish if there were any traces - such as the blood reportedly found in the McCanns' car - that would contradict, or raise questions about the accounts given by any of those involved in the case.
Other possibilities which have been studied are whether there is evidence of any other adults in the McCanns' apartment or whether there is evidence that the couple were present at a location not mentioned in their description of events.
The other objective of the police inquiry is to establish exactly what happened on 3 May when Madeleine vanished from the Mark Warner holiday complex in Praia da Luz.
According to the original version of events, the McCanns arrived for dinner at 8.40pm when they joined their friends Rachael Oldfield, her husband Matthew Oldfield, Jane Tanner, Russell O'Brien, David and Fiona Payne and an unidentified woman.
Mr McCann says that he then checked on Madeleine and her two year old twin siblings at 9.05 and that Matthew Oldfield then checked again at 9.30 before Kate McCann discovered the Madeleine was missing at 10 pm.
Mr Oldfield has since admitted that he only listened at the apartment door - and did not see the children - and detectives now will be seeking to establish who the last person was to see Madeleine - other than her parents.
Little attention has focused on this issue before because no doubts had been publicly aired about the McCanns'
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