Nottinghamshire Police has been asked to “urgently produce an improvement plan” after being put into special measures.
The force has been moved to an enhanced level of monitoring by watchdog His Majesty’s Inspectorate of Constabulary and Fire & Rescue Services (HMICFRS).
Inspectors said “the force needs to improve how it manages and carries out effective investigations” and “doesn’t have adequate processes, planning or governance arrangements in place”.
It comes after Nottinghamshire Police faced intense criticism over the handling of the case of paranoid schizophrenic Valdo Calocane, who stabbed to death three people in Nottingham last June.
His Majesty’s Inspector of Constabulary Roy Wilsher said: “We move police forces into our enhanced level of monitoring, known as Engage, when a force is not responding to our concerns, or if it is not managing, mitigating or eradicating these concerns.
“The Engage process provides additional scrutiny and support from the inspectorate and other external organisations in the policing sector to help the force improve and provide a better service for the public.
“Nottinghamshire Police has been asked to urgently produce an improvement plan and will meet regularly with our inspectors. We will work closely with the force to monitor its progress against these important and necessary changes.”
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The reasons behind the decision are set to be included in the latest inspection report due to be published later this year.
Nottinghamshire Police is now one of six forces in special measures, along with the country’s two largest, the Metropolitan Police and West Midlands Police, as well as Devon and Cornwall, Staffordshire, and Wiltshire.
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The force is being investigated by both the police watchdog the Independent Office of Police Conduct and the College of Policing over its handling of the Calocane case, while the victims’ families have hit out at its leadership.
Calocane was detained at a high security hospital for killing university friends Grace O’Malley Kumar and Barnaby Webber, both 19, and 65-year-old school caretaker Ian Coates, after prosecutors accepted his manslaughter plea due to his history of mental illness.
Last month, Mr Barnaby’s mother Emma Webber called for the chief constable Kate Meynell to step aside while allegations of failings are investigated after it emerged her son was among members of a police WhatsApp group in which graphic details were posted about the killings.
The Court of Appeal will decide whether Calocane’s sentence was too lenient.