Max Spiers, from Canterbury, died in mysterious circumstances during a trip to Poland, where it is believed he was going to reveal a major UFO cover up.
Polish authorities claim the 39-year-old – who was perfectly healthy when he left the UK – died of natural causes.
But a British coroner has launched a probe after a post mortem failed to determine how he died.
His supporters and his family are convinced he was bumped off by Satanists – or Nazis from space.
Sarah told the Sun: “He was terrified, he wanted to leave.
“He rang me secretly because they wouldn”t let him talk to me and he said he was trying to get away from them.
“They were doing very dark black magic and Satanic rituals to “de-programme” him and get rid of demons.
“He said they had given him something that had put him in a coma for two days.”
She added: “He said something demonic is going on, he said he needed to get out and find a church or a holy place – but two days later he was dead.
“They were doing some kind of black magic on him.”
His mum – who has maintained she is a “sane, normal person” – is also convinced that his body was tampered with while Polish authorities left him unattended overnight.
Vanessa said: “The police and the doctor all went off and left my son dead lying there with these people.
“I was on the phone to one of them and I could hear in the background there were strange suggestions of things that should be done.
“Things like ‘put milk by his side’ and ‘get garlic and put flowers and put vinegar’ and very weird goings on.
“I could hear it being said. It sounded like rituals going on. Some sort of Satanic rituals.
“Then I found out that his body hadn’t left the house until the following morning.”
She also claims her lawyers have logs proving police and doctors left him at the house where he was found overnight.
Vanessa – who has admitted she did not endorse her son”s conspiracy theories – also said Max had appeared “drugged” during his last interview with fellow truth-seekers.
Polish authorities had ruled Max had died from natural causes – but an autopsy conducted in Kent after his body was returned to Britain said the “results were inconclusive”.
A spokesman for North East Kent coroner”s office said: “The coroner has opened an investigation into the death of Max Spiers, which may or may not result in the need for an inquest.
“A postmortem was ordered and has been undertaken. However, the results were inconclusive and further tests are being carried out.
“When these results have been received, the coroner will decide whether to continue the investigation and, as part of that investigation, to hold an inquest.”