Police investigating the disappearance of Madeleine McCan have now identified a second person of interest in the case.
German investigators are hunting for the person who was on the other end of the phone to prime suspect Christian Brückner the night that Maddie went missing in 2007.
A key piece of evidence in the case is that a phone allegedly belonging to the convicted sex offender was 'pinged' by a cell phone tower in Praia da Luz on the night she was taken, and police are interested in whoever Brückner called.
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Explaining in ITV documentary Madeleine McCann: The Hunt For The Prime Suspect - which aired last night - public prosecutor Hans Christian Wolters explains: "We are now looking for the other person who was on the call." You can watch the video here:
Wolters added: "We want to know what they talked about. but we also want the other person to tell us who they were talking to.
"The data shows that one phone was there, not who used it. That's why the other person on the call would be so important to us."
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He added: "They could tell us who they spoke to, possibly our suspect. In that regard, it could result in a conviction."
Crime law specialist Dr Alexander Stevens went on to point out that the biggest challenge for prosecutors when it comes to witnesses is that "any confidante would have to incriminate himself."
Stevens added: "Once you are involved in a murder crime they would have to at least be sentenced to 10 years in prison so the chances of getting someone to come forward are actually quite low."
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German-born Christian Brückner was identified by police as the chief suspect in the case in June.
He is described as "a sexual offender that has been convicted several time also for sexual abuse against children," who lived on and off in the Algarve between 1995 and 2007.
Brückner was reportedly in the area where Maddie was last seen 13 years ago, before her disappearance.
Germany's federal criminal police office - who are working with the MET and Portuguese forces on the case - stated the man was currently serving a jail sentence for raping a 72-year-old woman, with two previous convictions for "sexual contact with girls".
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He was first questioned about Madeleine's disappearance in 2013 after a witness linked him to the crime but police could not form a case.
His name reemerged in 2017 when someone tipped off police claiming he had been boasting about Madeleine's kidnapping.
In June, police had appealed to the public for any information, calling specifically for information on two vehicles believed to be significant to the case: a 1980s model VW T3 Westfalia camper van and a 1993 British Jaguar, model XJR 6, with a German number plate.
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Three-year-old Maddie went missing in 2007 while holidaying with her family at a resort in Praia da Luz in Portugal's Algarve region.
The subject of her disappearance instantly gained global attention and is now considered the most heavily-reported missing persons case in modern history.
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