Dozens of people in the UK died after buying a poisonous substance from a seller in Canada, who is accused of helping people to take their own lives.
The National Crime Agency (NCA) could not confirm the chemical was the direct cause of the 88 deaths but has launched a criminal investigation.
British police have been making welfare visits to hundreds of addresses to trace buyers across the country.
Kenneth Law was arrested in Toronto in May, accused of assisting suicide.
On Friday he appeared briefly in court in Brampton, Ontario, before the hearing was adjourned to 8 September.
The 57-year-old is thought to have run a number of websites selling equipment to assist suicide.
Canadian authorities believe he sent 1,200 packages to customers in more than 40 countries, though it is not known how many included the poisonous substance.
BBC News is not naming the chemical allegedly sold by Mr Law.
Peel Regional Police said they began investigating the case in April following the sudden death of an adult in the Toronto area.
Alleged links between Mr Law and deaths in the UK first surfaced in an undercover investigation by The Times the same month.
Since Mr Law’s arrest, police forces across the UK have been making welfare checks on everyone who ordered the substance.
The NCA, which was coordinating the checks, said that 272 people in the UK had been identified as buying from Mr Law over a two-year period.
The NCA says 88 of them later died, but at this stage there are no confirmed links between the items purchased from the websites and cause of death.
NCA deputy director Craig Turner said: “Our deepest sympathies are with the loved ones of those who have died. They are being supported by specially trained officers from police forces.
“In consultation with the Crown Prosecution Service, the NCA has taken the decision to conduct an investigation into potential criminal offences committed in the UK. This operation is under way.”
Tom Parfett, from Maidenhead, was 22 when he ended his own life in October 2021 after buying the chemical from Mr Law.
His father David Parfett is angry at what he sees as police failures.
“It’s important for families to understand what has happened and why policing worldwide allowed this scale of deaths despite clear warning signs,” he said.
Mr Parfett fears there other suppliers out there and unregulated websites promoting suicide.
“What can be done immediately to close down internet sites that prey on vulnerable young people and prosecute the people who take pleasure in helping others take their own life?” he asked.
In a separate interview with BBC Radio 4’s Today programme, Mr Parfett said his son had discussed taking his own life with people he met online in communities set up to discuss the subject, and was encouraged to do so by some.
He added: “We have to accept that in the modern age, people can find like-minded people to discuss even the most difficult problems…those communities are unregulated and causing a huge amount of harm.”
Mr Parfett called for stronger action from police and policymakers to crack down on people selling poison, and for online communities targeting vulnerable people to be regulated.
He continued: “We need to be more sensitive around the risks that people like Tom have in society through their ability to find information online that is unchallenged.”
Under the country’s criminal code, counselling or aiding a person to die by suicide can result in a 14-year prison sentence.