The Independent Office for Police Conduct (IOPC) probe is also looking into complaints by the victims' families that there were "flaws" in the handling of the murder investigation and "missed opportunities" by Nottingham Police to prevent the deaths.

University students Barnaby Webber, 19, and Grace O'Malley-Kumar, 19, and school caretaker Ian Coates, 65, were stabbed to death by Calocane in Nottingham in the early hours of 13 June.

The killer was last month given a hospital order at his sentencing for manslaughter by diminished responsibility after Nottingham Crown Court was told he had been diagnosed with paranoid schizophrenia.

The watchdog said the probe "follows a voluntary referral by the force of complaints they have received from the victims' families".

The families allege "serious failures" in connection with the police probe into the killings and have welcomed the watchdog's decision to investigate.

Other complaints relate to the force not executing an outstanding warrant for Calocane's arrest before the killings, and concerns about its communication with the families.

In a series of missed opportunities to prevent the killings, Calocane had previously been detained in hospital four times, and a warrant for his arrest had been issued months before his deadly rampage.

Calocane had been wanted on a warrant in Nottingham for nine months, for an assault on a police officer that happened back in September 2021, according to defence barrister Peter Joyce.

The victims' loved ones expressed anger after the Crown Prosecution Service (CPS) accepted Calocane's guilty pleas to manslaughter on the basis of diminished responsibility.

An independent review has been ordered by the attorney general into the CPS's decision and whether it sufficiently consulted with the victims' families.

Barnaby's mother Emma Webber said the families did not meet the CPS and the police with regards to the plea until 24 November, shortly before Calocane's pre-trial hearing.

Barnaby's family, from Somerset, have described the hospital order as a "huge insult" and called for a public inquiry into the case.

Speaking on behalf of the relatives on Monday, Ms Webber accused the Nottinghamshire force of "serious failures... in connection to the police investigation into the brutal murders of our loved ones and the attempted murder of three other innocent individuals".

Calocane also pleaded guilty to the attempted murder of three people who were hit by a van stolen from Mr Coates.

On 2 February, the IOPC announced it had begun an investigation into the contact Leicestershire Police had with Calocane.

It emerged during Calocane's sentencing hearing that in early May last year - around five weeks before the killings - he was accused of attacking two employees at a warehouse in Kegworth, Leicestershire.