There is an expression in German, wie die Jungfrau zum Kind. Literally it translates as ''like the virgin to the child'' but it denotes a bolt from the blue, a piece of information that changes everything.
It was this phrase that flashed through the mind of Hans Christian Wolter - the German prosecutor in the Madeleine McCann case - when he first learned of the possible involvement of his countryman, 43-year-old Christian Brueckner in one of the most enduring mysteries of our time.
By then Brueckner was already serving a 15-month prison sentence for sexual abuse of a child in Germany.
After his release he fled to Milan and was arrested there and brought back to Germany to face charges relating to the rape of a 72-year-old American tourist in 2005.
In December last year he was sentenced to seven years for this offence, but the sentence has not yet been imposed because of an ongoing appeal. He could be released on parole as early as today after serving a short period of time for minor drug offences.
While this was unfolding, German police were investigating his possible involvement in the Madeleine McCann disappearance. They had been told about a conversation that Brueckner had had with a drinking friend of his in which he - Brueckner - claimed to know "all about" Madeleine McCann.
Brueckner was already known to have been living as a transient in the Algarve at the time of Madeleine's disappearance, but Goncalo Amaral, the man who led the investigation in Portugal, dismissed the possibility of his involvement in the case.
Until 2018 German authorities also had no reason to suspect Brueckner. "To say we were surprised is an understatement," Wolter says.
The investigation has centred on two vehicles - a Jaguar and a VW camper van - known to have been used in Portugal by Brueckner, as well as a 30-minute phone call he took in the town of Praia Da Luz, an hour before Madeleine went missing.
"We want to speak to someone who knows these phone numbers and who can tell us what he was doing around this time when the calls were made, whether he was there with a small child, whether there were any other striking observations," Wolter says.
"The problem for us was that in 2018, when we became aware of this person's potential involvement in the case, we could no longer access the phone data from 2007, and our British colleagues helped with this. They gave this phone data to us - and without this we wouldn't have been able to make the appeal this week.
"We don't know if the person or persons with whom he made the calls were German themselves, or not."
For more than 25 years Brueckner moved between Germany and Portugal, often escaping various criminal proceedings against him for the sexual abuse of children, but also drug-dealing and falsifying documents.
He was given a two-year youth sentence in 1994 for sexually abusing a child when he was just 17. In 2013 he abused another young girl and was caught with child pornography, and was jailed for those offences in 2016.
Wolter says that his office is hopeful that further victims of Brueckner may be able to shed light on his movements in the Algarve.
"We're working on the assumption that there were more victims than Madeleine, and these wouldn't necessarily have to be children, they may also be adults that were in some way sexually attacked by this man. What we're hoping is that these people, who haven't had the strength to speak out, may now be able to do so.
"We hope with the passing of time that these people, if they weren't killed, understand that they are not alone, that there's nothing that he can do to them, because he's in prison."
Although Scotland Yard is still treating it as a missing person case, Wolter says his office is working on the presumption that Madeleine was murdered.
"We have no reason to believe that the girl is alive and because of this we have to work on the assumption that she is dead. It has never happened here that a child has been found again after being missing for so long. It is of course conceivable that this could happen, it may have happened elsewhere, but it is very unlikely."
Madeleine's family were made aware some time ago that German authorities were treating it as a murder investigation, he adds.
Wolter says that if German authorities can assemble enough information to begin a prosecution, that will take place in Saxony-Anhalt, the state in which Braunschweig lies.
"The way it works in German criminal law is that we can prosecute individuals who've committed sexual offences against children in Germany - whether they are German or not - but we can also prosecute German individuals who have committed these kinds of offences abroad.
"If - and this is still hypothetical - we can get this investigation to the stage where we can formally bring a case against this individual, then that case will proceed here before the state court in Braunschweig."
Although Brueckner was named in a number of media outlets including Irish, Portuguese and British, Wolter says that there were good reasons not to give his identity in the press conference, which was held earlier this week.
"We didn't publicise the name or the picture of the accused and there were two reasons for this," he explains.
"Firstly we want to protect the accused - if he turned out not to have been the person who committed this crime it would have been the wrong thing to do to put his name out there because with crimes like child sexual abuse or murder even if the investigation against him were to be discontinued there would still be a suspicion hanging over this man.
"Secondly, we also want to be able to establish that if someone reaches out to us based on the information we've given that they really know who the person is. If they knew who he is, just from the information we've given then we will know that the information they're providing will be especially important for us."
More than five million Germans watched the press conference in which authorities there appealed for more information from the public.
Wolter says that in his 17 years with the state prosecutors office, he has never seen such interest in a case.
"The pressure has come mainly from the media who want to know as much as possible about the case but, from my perspective, we can continue our work quietly here. We're not coming under pressure from politicians or from above. The prosecutor's office is part of the executive in Germany but we do retain a high degree of independence."
Despite the evidence assembled against Brueckner, Wolter says German police are still dependent on the public for the last piece in the puzzle.
"How certain are we? Well, let me put it like this: we don't currently have enough to arrest the man or to bring a prosecution against him - but we do have very good reasons to believe he could be the offender.
"We're at the point now where we can't go any further with the normal methods at our disposal and we needed to appeal to the public in the hope that we can move the case on.
"Somebody out there knows something - and we hope that person can now come forward."
Madeleine's parents Kate McCann and Gerry McCann have said: "We will never give up hope of finding Madeleine alive, but whatever the outcome we need to know as we need to find peace."