The man suspected of abducting Madeleine McCann insists he was many miles from the scene having sex in his camper van with a young woman who will back his alibi.
Christian B says he drove the woman to the airport at Faro, Portugal, for a flight home the next day and they were stopped and photographed at a police roadblock.
The suspect, a German drifter, claims she was arrested at airport security for carrying an illegal pepper spray and later appeared in court.
Christian B, as he is known under German privacy laws, believes Portuguese police must have a record of those events that will establish his relationship with the young German woman who was on holiday with her parents.
Apparently, German police found a photograph of the woman lying in his camper van during their investigation into a rape for which Christian B, 44, is now serving a seven-year jail sentence in Germany.
When he first spoke of his alibi he couldn't recall the woman's full name, but it's understood he has since been able to identify her.
If it's true, the alibi would contradict vital, but circumstantial, evidence from mobile phone data masts that police say puts him close to the apartment from where Madeleine vanished from her bed 15 years ago in Praia da Luz on May 3, 2007.
According to police, the suspect's mobile phone was in the village when it received a call from 7.32pm to 8.02pm.
Madeleine's parents said she disappeared between 9 and 10pm. Christian B insists that by 10pm he had driven miles along the coast east towards Faro.
German prosecutor Hans Christian Wolters, who is leading the Madeleine investigation with Portuguese and British detectives, told Sky News: "I assume if he has anything that exonerates him that sooner or later he will share it with us and we would then check it out. What happens then, let's see.
"So far he has told us nothing, he's given us no alibi. So, we can only work on the evidence we have found so far in our investigation. And there was nothing to exonerate him."
But German prosecutors had never, until two weeks ago, formally interviewed Christian B, a convicted sex offender, even though he has been their chief suspect for more than four years.
They questioned him on 21 April on behalf of the Portuguese authorities who had just made him an arguido, a formal suspect, for their own purposes to avoid a statute of limitations on serious crimes under Portuguese law.
But as an arguido Christian B had the right to silence and refused to answer questions about his whereabouts on the night Madeleine vanished.
His former lawyer Serafim Vieira, who represented him over the theft of fuel in Portugal in 2006 said the suspect's alleged alibi should not be simply dismissed.
He said: "If he really does have that alibi he should work on that. What I know about him, he was a straight person so if he says he has an alibi that is possible."
Madeleine was nearly four when she disappeared from the family's rented holiday apartment in Praia da Luz on the Algarve coast.
Her parents Kate and Gerry McCann were dining with friends nearby on the holiday complex and had left her sleeping with her younger twin siblings. They and their friends were taking turns to check on the sleeping children every half-an-hour.
The McCanns were once suspects for their daughter's disappearance, but were later cleared of all suspicion. They cling to the hope she will still be found alive.
On May 3 it will be exactly 15 years since she vanished - and her parents released a new statement to mark the date.
In it they talk about their search for the truth, saying: "Uncertainty creates weakness; knowledge and certainty give strength, and for this reason our need for answers, for the truth, is essential."
They thank investigators and add: "Many people talk about the need for 'closure'. It's always felt a strange term. Regardless of outcome, Madeleine will always be our daughter and a truly horrific crime has been committed."
Portuguese police and UK detectives have investigated, but the German authorities took the lead in 2017 when a friend of the suspect told them Christian B had allegedly claimed to know what had happened to Madeleine.
Mr Wolters said: "We actually haven't found a single piece of the puzzle in the two years that would have somehow helped to exonerate Christian B. So it's really nothing that would somehow maybe be an alibi or something. Nothing of that has come to light, really at any point.
"What we found out, it all went in the other direction, so was rather incriminating, without me being able to elaborate on that now.
He added: "However, it is not foreseeable when we will come to an end. So I can't say that we will definitely conclude the investigations this year. That really also depends on how this develops."
Mr Wolters has said in the past he believed Madeleine was dead, but would not reveal what evidence he had.
He is also investigating Christian B over three other sex assault allegations, including the rape of a young Irish woman, Hazel Behan, who was working as a holiday rep on the Algarve in 2004.
She has spoken publicly about being raped in her apartment at Praia da Rocha and waived her right to anonymity.