Nicole Fehlinger’s father, Dieter, returned to speak to the German equivalent of the FBI for the second time.
Prosecutors confirmed that he gave a lengthy interview and that they are seriously considering this new lead.
Madeleine disappeared from her family’s vacation apartment in Praia da Luz, Portugal, in May 2007, days before her fourth birthday.
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Nicole was living in Portugal at the time with Brueckner, who has been identified as the prime suspect in Madeleine’s disappearance.
Mr Fehlinger told investigators that Nicole and Brueckner had a secret resort in the Algarve that no one else was aware of.
He said he was guarded by four fierce Kangal dogs, who have the strongest bite in the world.
Mr Fehlinger said: “No one knows that my daughter not only had the private residence in Portugal, but also a separate complex.
“No one has ever searched there or the property has ever come into contact with Maddie, Brueckner or Nicole.
“It’s an overgrown property of over 5,000 square meters. It was a secure area, surrounded by fences, and four aggressive Kangal dogs were there day and night.
“The area is completely overgrown, not visible. No one could go in or out, only my daughter and maybe Brueckner with her.
The secret complex sits on land very close to the house where Nicole lived in the Algarve, 38 miles from Praia da Luz.
The mother of three was part of a reception program in Portugal for young Germans in difficulty. There is no suggestion that she has anything to do with the disappearance of Madeleine, from Rothley, Leicestershire.
German police are only investigating Brueckner in the case. He is currently in prison after being convicted of other offenses, including rape.
Last year, they named Brueckner, 44, as the main suspect of the missing Madeleine after phone records placed him in the complex the night she went missing.
The pedophile accused prosecutors of bringing “shame” to the German legal system by making “scandalous” statements against it.
Nicole, also 44, was in a relationship with him for a few years after they met at a bar. Court documents in Portugal revealed they had regular phone conversations and spent a weekend in Lisbon together just months after Madeleine disappeared.
Nicole worked as a hotel receptionist before returning to Würzburg, Germany about 10 years ago.
It is not known whether they continued their relationship when Brueckner returned to Brunswick to open a bar in 2012.