Jenny Murat (71) revealed she had unearthed a new line of inquiry when she and a friend reported a gang of suspicious strangers to police - days before she knew her son was in the frame for the shocking crime.
But Jenny claimed the lead - centred on a house less than two miles from where Madeleine was grabbed - was IGNORED for weeks by detectives.
And she accused them of not acting on it urgently because Murat was their only suspect and she is his only alibi.
Sister-in-law Sally Eveleigh, 56, told The People, that as a huge manhunt was launched for the kidnapper, Jenny remembered seeing three strangers - a couple in their 30s and a woman in her 50s - at a large isolated house nearby.
She had spotted the building - called Casa Ao Pescareza - while walking her dog along a coastal path.
Sally said: "Jenny was intrigued because they looked like gipsies or travellers and were driving a clapped-out old red car with a British number plate. They looked out of place in this grand house with its big drive and swimming pool.
"But they also seemed to have a string of visitors - men in smart suits driving plush new cars like Mercedes.
"On one occasion the older woman told Jenny they always came to Luz in summer and were staying four months.
"She spoke perfect English but Jenny thought she had a French or possibly Middle-Eastern accent.
"Then two days after Maddie vanished Jenny was walking down that way again and noticed the house was shut up and deserted.
"There was no sign of the trio after that - even though they had said they would be there all summer."
Jenny jotted her recollections on a piece of paper which was passed to police.
She also gave them the name of a pal who had shared her suspicions about the "odd" tenants.
The People, which has seen Jenny's notes, has learnt cops did not contact the friend for two months.
Sally said: "People will probably say that Jenny is clutching at straws.
"But she reported it to police a couple of days after Madeleine vanished and more than a week before anyone knew Robert would be questioned."
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