A 43-year-old German prisoner who travelled around Portugal in a camper van is now the focus of Scotland Yard’s investigation into the disappearance of Madeleine McCann.
Police believe he was in the area where the three-year-old was last seen in May 2007.
They are appealing for information about the van and the suspect’s other vehicle, a Jaguar.
The man transferred it to someone else’s name the day after she vanished.
“Someone out there knows a lot more than they’re letting on,”said DCI Mark Cranwell, who’s leading the Met inquiry.
The force said it remained a “missing persons” investigation because it doesn’t have “definitive evidence” as to whether Madeleine is alive or not.
However, German investigators at the Federal Criminal Police Office, the Bundeskriminalamt (BKA), have classed it as a “murder inquiry”.
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Scotland Yard said the German authorities had taken the lead on this aspect of the case because the German suspect was in custody in their country.
Detectives said he was in jail for an “unrelated matter” and had “previous convictions”, but they declined to supply more details.
An appeal on German television was broadcast this evening at 19:15 BST.
DCI Cranwell said the prisoner, then aged 30, frequented the Algarve between 1995 and 2007, staying for “days upon end” in his camper van and living a “transient lifestyle”.
He was in the Praia de Luz area where the McCann family was staying when she disappeared and received a phone call at 7.32pm, which ended at 8.02pm.
Police have released details of the suspect’s phone number and the number he dialled saying any information about them could be “critical” to the inquiry.
They also want the person who called the suspect to come forward.
“They’re a key witness and we urge them to get in touch,” said DCI Cranwell.
“Some people will know the man we’re describing today… you may be aware of some of the things he’s done,” he said.
“He may have confided in you about the disappearance of Madeleine.
“More than 13 years have passed and your loyalties may have changed,” he added.
“Now is the time to come forward.”
Police said the suspect was one of 600 people that detectives on the inquiry, known as Operation Grange, originally looked at, though he had not been a suspect.
After an appeal in 2017, “significant” fresh information about him was provided.
Since then, Met detectives have carried out “extensive inquiries” in Portugal and Germany in order to gather more details about him.
Scotland Yard said they were trying to “prove or disprove” his involvement in the case and retained an “open mind”.