Three Leicestershire Police officers are being investigated by the police watchdog for their contact with Valdo Calocane before he killed three people in Nottingham.
Two police constables and a sergeant may face charges of potential misconduct regarding how they investigated an earlier case against Calocane.
Barnaby Webber (19), Grace O'Malley-Kumar (19), and Ian Coates (65) were fatally stabbed in Nottingham on June 13, 2023. While Webber and Kumar were Nottingham University students, Coates was a caretaker at Huntingdon Academy.
The force had referred itself to the Independent Office for Police Conduct (IOPC), after it was found that officers were investigating Calocane's alleged assault of two colleagues on May 5 at a warehouse in Kegworth, Leicestershire.
Investigators are looking at what information was available to police about Calocane before the triple murder.
The police claimed that nobody was seriously hurt, and Calocane was escorted off by the security guards before they reached the spot.
The IOPC is also examining Nottinghamshire Police's decisions and actions relating to Calocane between September 2021 and June 2023.
The police watchdog said it would also probe how the force informed the families of the death of their loved ones and the subsequent support they provided them. This includes how they were informed about the ongoing investigation.
Senior officers are also being probed to see whether relevant policies were followed. All Nottinghamshire officers are currently being treated as witnesses, the IOPC said.
The watchdog is separately examining several complaints made by the Nottingham victims' families.
Calocane was sentenced to detention in a high-security hospital after admitting manslaughter on the grounds of diminished responsibility.
His guilty pleas were accepted after medical evidence showed he had paranoid schizophrenia.
During his sentencing, a judge told Calocane that he would probably be detained in a high-security hospital for the rest of his life.
The families of the attack victims expressed their anger after the killer was given the hospital order.
A review launched by the attorney general earlier this year found the sentence was not unduly lenient.
Bob Blackman MP speaks during the event