The family members - aged 80 and 53 - along with an 81-year-old woman have been charged over the alleged assisted suicide of a Gold Coast man in April.
The elderly duo supported the 53-year-old at an end-of-life business which provided the victim a veterinary euthanasia drug sourced under false pretences, police allege.
The investigation has been widened to include other deaths dating back to 2021, with "around or greater than 20" to be probed including some in Victoria and NSW.
"The alleged offender is deliberately targeting vulnerable people, taking advantage of them in their most desperate moments," Detective Inspector Mark Mooney said of the 53-year-old on Monday.
A coroner ordered a police investigation after the Gold Coast man's post-mortem revealed a lethal dose of the life-ending drug for animals - pentobarbitone - as the 43-year-old's cause of death.
The drug is tightly controlled in Queensland and police charged the trio after an extensive probe into the man's health record and medical treatment.
The trio were not known to the 43-year-old, however, the deceased's family had contacted the business, police allege.
The 43-year-old had suffered from medical conditions "for a long period of time" but did not qualify under Queensland voluntary assisted dying laws which came into effect in January 2023.
"Through our investigations, we've been able to establish that the two older parties are involved through the 53-year-old male," Det Mooney said.
"They support him carrying on his business, this End of Life service.
"We will allege he (the 53-year-old man) hasn't acted compassionately at all - this is a business transaction for him."
The trio are not veterinarians but police alleged the 53-year-old had sourced the euthanasia drug "under false pretences" by setting up a charity and accessing it through a lawful supplier.
"That charity is just a front," Det Mooney said.
Police are asking anyone who has engaged with a business called End of Life Services to come forward.
"Queensland has very strict legal processes in place for people who are considering end-of-life options," Det Mooney said.
"These safeguards exist to ensure proper oversight, consent and protection for those involved."
The 53-year-old Main Beach man has been charged with two counts of aiding suicide and one count each of trafficking in dangerous drugs, possessing dangerous drugs and receiving or possessing property obtained from trafficking or supplying.
His 80-year-old father from Ashmore was charged with trafficking and possession of dangerous drugs.
The woman was charged with aiding suicide, trafficking dangerous drugs, possession of dangerous drugs and the sale of potential harmful things.
They are set to appear in Southport Magistrates Court later in September.
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