Mr Spiers, 39, a dad-of-two from Canterbury, was found dead on a sofa at the home of Monika Duval’s, where he stayed home during a trip to Poland.
The science fiction writer is said to have been with him at the time and phoned the emergency services.
Warsaw prosecutor Lukasz Lapczynski said the nature of the relationship between Mr Spiers and Ms Duval remains unclear.
He told the BBC: “The nature of this relationship will be determined during the investigation.
“The doctor started resuscitation which was not successful.
“As a result of the doctor’s decision, the police weren’t involved in conducting additional procedures.”
The prosecutor said he plans to interview everyone who was present when Mr Spiers died in Warsaw.
He added: “The information about Max Spiers’ death reached the prosecution office on August 30 when the body had already been transported to the UK.
“We couldn’t do a post-mortem which is essential in such cases.”
Mr Spiers was in Poland in July to talk about conspiracy theories and UFOs, of which he claimed to have sensitive knowledge.
His mum Vanessa Bates, 63, is convinced foul play is involved.
Just before his death he texted her: “Your boy’s in trouble. If anything happens to me, investigate.”
She claims he could have been sacrificed by a Satanic cult he was about to expose who slowly poisoned him to keep him quiet.
Four days before his death, Mr Spiers revealed he was investigating a Satanic child abuse scandal at 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 a sexually transmitted disease, surfaced from the centre.
Mr Spier’s had a fiance in the UK, Sarah Adams, 31, a fellow conspiracy theorist.
She claims he was held prisoner by a Satanic cult just days before he died.
She said: “He was terrified, he wanted to leave.”
Ms Bates also said he had been investigating prominent government leaders and celebrities before he died.
An inquest into Mr Spiers’ death was opened in Kent in December and is set to continue later this year.