Disturbing behaviours Madeleine McCann prime suspect Christian B displayed before arrest

Warning: This article contains discussion of sexual assault which some readers may find distressing.

Christian Brueckner, the prime suspect in the Madeleine McCann disappearance case, displayed some odd behaviours in his earlier years according to some of those close to him.

Known as the most famous missing child possibly in the world, it's been 18 years since McCann vanished yet earlier this year, the investigation was re-opened with authorities determined to get to the bottom of it.

It was revealed on Monday that Brueckner 'refused' to be interviewed by the Metropolitan Police ahead of his release from prison today (17 September), as his seven-year sentence came to an end.

He was jailed after being found guilty of the rape of an elderly American woman in Portugal in 2005, though he was never charged with any crimes linked to McCann.

But a new ITV documentary is set to delve into the life of Brueckner, taking a look at what may have caused him to kidnap the young Brit, or if he really is a psychopathic serial killer.

Madeleine McCann has been missing for almost two decades (PA)

Christian Brueckner, also known as Christian B, is the prime suspect in McCann's disappearance according to German prosecutors, who claim that they believe he is responsible for kidnapping her, though there are still no charges against him ahead of his prison release.

Dr Graham Hill, a former senior police detective who was part of the early stages of the investigation, returns to Praia de Luz in Portugal for the first time in this documentary, going through Brueckner's criminal history in Europe and interviewing people close to him to get a better understanding of the suspect.

McCann, who was three at the time of her disappearance, was on holiday with her family in the Portuguese resory when she vanished on 3 May 2007, which has remained a mystery for almost 20 years.

While the German has long denied any involvement in her disappearance, many people who met the man seem to think that he may have had the capabilities to pull it off.

Brueckner was adopted as a child and suffered abuse from his adoptive father, forcing him to move into a children's home by 15.

Here, one of the boys who was in the children's home in the early 1990s, Thomas Hurtle recalled how the convict behaved towards others.

Brueckner claimed that McCann was 'dead' just years after her disappearance (Alexander Koerner/Getty Images)

While he'd throw bottles at people and attack carers, to the point where they'd hide in their room, it was a discovery in Brueckner's room which really haunted people.

"There were preserving jars with liquid inside, I thought they might be dead animals or some kind of inards," he would recall, as nobody was allowed into his room up to that point.

He would then become a convicted sex offender at 17, exposing himself for a six-year-old, before exposing to himself as a nine-year-old while on remand for the first offence.

Later on in his life, he would start to get involved in the criminal underworld, modifying vehicles to transport drugs.

A man who he worked with explained in the documentary recalled that Brueckner boasted about the space in one of his vans, quipping 'it's so big you could hide a small child inside'.

While selling drugs in Portugal, he would often stay in isolalted houses or on a caravan away from the campsite - a man named Ken, who camped at the site, recalled seeing him there.

When asked by Dr Hill if he would be surprised to learn of Brueckner's attraction to children, he replied: "Well, the way he looked at girls and young girls, he's a predator, more than a paedofile, he's a pervert."

"He should have been made a suspect three hours after she (McCann) went missing," he added.

Brueckner's behaviour has been questioned by many over the years (Carabinieri Milano via Getty Images)

Four years on from McCann's vanishing though, Christian B ran a small kiosk in the German city of Braunschweig.

While he chose to wear a sport jacket despite hanging out with criminals, an expert said he wanted to 'look like the important person', but this wasn't accurate.

One of the kiosk workers recalled to experts that Brueckner 'flipped out' when the case was running on the TV, as she shouted out loud: "The girl is dead!

"Yes, pigs can also eat human flesh, you know," he bizarrely said towards the TV.

Other allegations from his time of the kiosk also included him targetting the child of a woman he was dating, as is stated in the documentary.

German prosecutor Hans Christian Wolters himself claims of the suspect: "In the last five years, our investigators have not resulted in an exonerating evidence of Christian Brueckner being involved, he has no alibi, there is only evidence that incriminates him.

"He is the prime suspect and above all, he is the only suspect. We really only have evidence that points to him as the perpetrator but as to whether we can prove it in a court of law, that is the big task that lies ahead of us," he stated.

Madeleine McCann: Searching For The Prime Suspect airs Wednesday 17th September at 9pm on ITV1 and ITVX.