Families of the Nottingham attacks victims said police did not tell them a man who had taken an interest in them had a history of violence when he attempted to visit them.
Barnaby Webber, Grace O'Malley-Kumar and Ian Coates were fatally stabbed by Valdo Calocane, who had been diagnosed with paranoid schizophrenia in 2020, on 13 June 2023. He tried to kill three others.
Grace's father, Dr Sanjoy Kumar, said he was told in 2025 that a man had visited his GP surgery in London. Emma Webber, Barnaby's mother, said he had also travelled to Somerset, but neither were informed of the man's history of mental ill health.
Nottinghamshire Police said it had received a complaint and referred the matter to the Independent Office for Police Conduct (IOPC).
The IOPC confirmed it had received a referral of a complaint from Nottinghamshire Police in relation to information the force allegedly withheld from the families of Calocane's victims about a potential witness who had tried to contact them.
Kumar told the BBC he was informed by Nottinghamshire Police in 2025 that a man from Nottingham, who had taken an interest in his family and the Webber family, had visited his surgery in London.
He said police gave little to no details about the man, with no mention of his past mental health history.
"They [the police officers] said it was a very lowbrow thing. There was no assessment of risk to me or my family," he added.
"They didn't mention the word 'risk'."
Kumar said it was only recently, as a result of ongoing investigations into Nottinghamshire Police, in relation to the attacks, that he was informed of further details about the man by the IOPC.
"Later, we found out from the IOPC that this guy was on the Prevent register, that he had been sectioned a few times or that he was known to be a violent offender," Kumar said.
"I'm angry. Don't place my wife, don't place my other child at risk. Do these people not realise the tragedy we've been through?
"Just be transparent. That's all we wanted."
Speaking to BBC Radio 4, Grace's mother, Dr Sinead O'Malley-Kumar, said she was "absolutely flabbergasted" that police were aware of the man's behaviour in December 2024, but did not inform families until about a year later.
"And what's more concerning is that, as victims, we weren't given the proper information," she added.
"We do not know what this man looks like. Should he ring on our doorbell? Should he come to our place of work again?
"The information has been minimal."
The Times reports, external Barnaby Webber's parents were not told for 18 months that a stalker had travelled to Somerset to try to find them.
Emma Webber, the mother of Barnaby Webber, said in a social media post she was "furious".
"Now we learn that another mentally ill man with a fixation on us and the Nottingham case travelled to Somerset, including our home town, seeking out victims' families; and the police chose not to tell us," she said.
Calocane pleaded guilty to three counts of manslaughter on the grounds of diminished responsibility, and to three counts of attempted murder in November 2023.
He was sentenced to an indefinite hospital order in January 2024. A public statutory inquiry, which is examining the lead-up to the attacks and decisions taken in the aftermath is set to resume in September.
What later emerged was a series of failings by authorities, including the police and mental health services - which Calocane was known to - prompting calls for a statutory public inquiry.
The Nottingham Inquiry heard from more than 100 witnesses as the lead-up to the attacks, the investigation that followed and the aftermath were examined.
