Madeleine McCann has been missing for 12 years. Now a new "masked man" suspect has emerged.

In the 12 years since Madeleine McCann’s disappearance, the theories have swirled, yet little has been uncovered to decipher what actually happened to the toddler who vanished from a Portuguese hotel room.

It’s a story that has captured the world, becoming the most publicised missing person’s case in modern history, as distraught parents Kate and Gerry McCann continue their search for Maddie.

Now – a key suspect has emerged with a horrific criminal past.

Serial child killer and paedophile Martin Ney, dubbed the “masked man”, is being questioned in line with the case, as he is believed to fit the description of a man acting suspiciously in the hours before Madeleine vanished in 2007.

The German 48-year-old was jailed for life in 2012 for murdering three children, and sexually abusing dozens more, The Sun reports.

On May 3, 2007, Maddie was sleeping in her family’s holiday apartment with her twin siblings, while her parents dined 55 metres away at a tapas bar in the complex with friends.

She was discovered missing by her mother, Kate McCann, late that evening during a routine check of the children.

Here is the trailer from the eight-part Netflix documentary released this year into the case of Madeleine McCann. Post continues after video.

The Portuguese police officer who first led the hunt for Madeleine, Goncalo Amaral, suggested the new lead could prove fruitful to the case, and perhaps close the book on what happened to Maddie.

“Detectives are preparing the end of the investigation, with a German paedophile who is in prison right now.”

Ney, a former youth worker, would dress in black and don masks and balaclavas while carrying out his attacks.

The recent Netflix documentary on the case featured a suspect who targeted British holidaymakers’ apartments in Praia da Luz in 2007, which seemed to match Ney's profile.

The man portrayed in the series had an accent, and preyed on children in a surgical mask. He replied “Yes” when one girl woke up and asked if he was her daddy.

Ney had traveled widely in Portugal in the 1990s, so knew the area from which Maddie disappeared well.

He was jailed for killing three German children — Stefan Jahr, 13, in 1992, Dennis Rostel, eight, in 1995, and Dennis Klein, nine, in 2001.

He is also suspected of the murder of a child in Holland in 1998 and another in France in 2004.

The timings raise the possibility Ney murdered a child every three years – 1992, 1995, 1998, 2001 and 2004, before Madeleine vanished in May 2007.

The reports come on the 12th anniversary of Madeleine’s disappearance, with Metropolitan Police Commissioner Cressida Dick stating on Thursday the force was pursuing “active lines of inquiry”.

The British police launched their own investigation in 2013, called Operation Grange, after a Portuguese inquiry failed to make progress.

It's believed they are now trying to establish if Ney was in the Algarve around the time of Madeleine’s disappearance.

Madeleine’s parents, Kate and Gerry, both doctors, have pledged never to give up the search for their daughter.

In a statement on Friday, they said: “The months and years roll by too quickly, Madeleine will be 16 this month. It’s impossible to put into words just how that makes us feel. There is comfort and reassurance though in knowing that the investigation continues and many people around the world remain vigilant.

“Thank you to everyone who continues to support us and for your ongoing hope and belief.”

For more on this topic:

How a nightmare about Madeleine turned Kate McCann into police's prime suspect.

Inside the sex trafficking theory linked to Madeleine McCann’s disappearance.

What we know about the twin siblings of Madeleine McCann.

“This could be the breakthrough”: Madeleine McCann Netflix documentary claims she’s still alive.