He vomited two litres of a black fluid before he died, his inquest heard in December.
Involuntary manslaughter
Now prosecutors have opened an investigation into involuntary manslaughter and want to speak to his girlfriend, who was present at the time of Mr Spiers's death.
Lukasz Lapczynski, prosecutor in Warsaw, said: "At this stage we have determined that the ambulance arrived at the request of Mr Spiers's partner.
"Our initial investigation indicates that it was Mr Spiers's partner [who called the ambulance], but the nature of this relationship is unclear."
Speaking to the BBC, he added: "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 information about Max Spiers's 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.
"The prosecutor told me he plans to interview everyone that was present when Max died. We know Monika was there, but it's unclear who else was around."
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When the conspiracy theorist's body was transported to the UK, his laptop and phone weren't among his belongings.
His mum Vanessa Bates said science fiction writer Monika sent the valuables a few weeks later but there was no Sim card in the phone.
'Somebody had wiped everything'
She added: "There was nothing on the computer at all. Somebody had clearly, definitely wiped everything off it.
"It makes no sense to wipe somebody's computer, no sense to me."
Ms Bates, who had previously stated she believed her son was "murdered by enemies", added: "I spoke to Monika for probably a couple of weeks [since Mr Spiers's death]. She was kind, she lived in a lovely house.
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"She said to Max 'Come stay here for a while, I've got a couple of good business things that we can get going'.
"I don't know what happened, but I know there's been a lot of people suggesting a lot of things and pointing a lot of fingers in her direction, and I'm sure that she wants to go very quiet."
Mr Spiers's inquest opened in December and continues later this year.