Max Spiers, 39, was reportedly trying to expose government cover-ups before he was found dead on a couch in Poland on July 16.
It is believed he was due to speak at a conference there and expose black magic and conspiracies related to celebrities, politicians and businessmen, Mirror UK reported.
The Canterbury coroner’s court revealed the father-of-two had allegedly spewed black liquid before he died.
Coroner’s investigations officer Caroline O’Donnell said in a statement: “When they returned, Mr Spiers became ill with a high temperature and was weak,” the newspaper said.
“The following day, Mr Spiers vomited two litres of black fluid. The friend called a doctor who attempted resuscitation before pronouncing him dead.”
Polish authorities had previously ruled his death was due to natural causes.
However, his mother, Vanessa Bates, claimed there was no post-mortem examination carried out on his body by local authorities at the time.
Days before his death, Spiers sent his mother a chilling message that said: “Your boy’s in trouble. If anything happens, investigate.”
Ms Bates and hundreds of other people believe his investigations into UFOs and government cover-ups may be the reason behind his alleged murder.
“But I think Max had been digging in some dark places and I fear that somebody wanted him dead,” she said.
Coroner Alan Blunsdon hopes to secure funding to have Spiers’ laptop and mobile phone forensically analysed.
Spiers had many theories, believing that aliens existed and also that humans were controlled by an elite secret society.