Max Spiers, 39, who is originally from Canterbury in Kent, travelled to Poland in July to give a talk on conspiracy theories, but was found dead on a sofa in the flat he was staying in.

Friends claim he died in his Warsaw apartment after vomiting "black liquid".

His family believe his death was suspicious after the father-of-two reportedly sent his mother, Vanessa Bates, 63, a text message two days before he died, which said: "Your boy's in trouble. If anything happens to me, investigate."

In a rambling interview with Polish YouTube channel PorozmawiajmyTV, during which he appeared to slur and complained of being tired a number of times, Mr Spiers talked of his enquiries into the Presidio Child Development Center run by the US Army in San Francisco.

A well-documented scandal broke in 1986, when allegations of suspected ritual abuse involving 60 children, including four children had that contracted an std, surfaced from the centre.

But, despite so many victims, only one man, Gary Hambright, a civilian employee of the centre and baptist minister was charged on 10 counts of child abuse.

The San Jose Mercury News reported in 1988 on claims Lt Col Michael Aquino, who was based at Presido, and the self-confessed founder of a Satanic church called The Temple of Set, was involved in the abuse, but despite a police inquiry, no charges were ever brought against him.

The case against Mr Hambright was also subsequently dropped after a judge ruled the allegations were too vague.