The police watchdog has found no misconduct from Essex police in a case where a woman died after being stabbed more than a hundred times by her mentally ill partner and officers took almost three hours to respond to a 999 call.

The attacker’s parents, who made the call but were unable to go to the scene because they were overseas, expressed dismay at the ruling.

The Independent Office for Police Conduct (IOPC) said the response was delayed for such a long time because the call had been wrongly categorised by the initial handler.

In January, a jury at Chelmsford crown court found Jake Neate killed his girlfriend, Suzanne Brown, 33, by stabbing her 173 times at the couple’s home in Braintree, Essex on 15 December 2017.

The findings followed a trial of the facts because Neate, who has schizophrenia, was deemed unfit to stand trial. The court heard Essex police officers arrived at the property almost three hours after Neate’s mother, Jan Neate, had first called them. By that time, it was too late to save Brown. The attack began on the evening of 15 December. Brown died in the early hours of the following day.

Jake Neate. Photograph: Family handout

Brown raised the alarm by calling Jan Neate in a distressed state when Jake Neate, who had never been violent before, began to attack her. Jan Neate then made an emergency call to Essex police. Questions have been raised about whether Brown would have survived had police arrived at the couple’s address soon after they received the call at about 9.30pm.

Jan and her husband, John Neate, had recently retired to Spain so were not able to quickly go to the couple’s home.

Jan and John Neate made complaints to the IOPC about procedures of emergency call handlers in the control room and the delay in police officers arriving.

They said they were “very disappointed” with the IOPC’s report. Two members of staff in the control room have received management action and a member of police staff from the control room has also received management action in relation to asking sufficiently probing questions.

The IOPC has asked Essex police to make call handling improvements. The watchdog said control room staff should be notified when emergency calls go past their target response times and it has asked police to provide callers with an assigned grading and estimated attendance time to urgent incidents, in line with national guidance.

The IOPC regional director Sarah Green said: “Our investigation found that the initial call to the force control room had not been categorised correctly as a domestic incident. This meant associated processes, including risk questions being asked that may have aided the response, were not triggered.”

Jake Neate was diagnosed with paranoid schizophrenia at the age of 20 while studying geography at Birmingham University. He began taking antipsychotic medicine and in his mid-20s was prescribed clozapine, which he responded well to. He had neither a history of violence nor a criminal record.

When Neate’s parents left the UK to settle in Spain a few months before the killing, things seemed to be going well for their son and his girlfriend. They had been living happily together for 10 years and were thrilled to have recently moved into a flat of their own.

Deborah Coles, the director of Inquest, which is providing support to Jan and John Neate, said: “This case once again highlights failures of the police to respond effectively to serious concerns raised about the risk to life of a young woman as a result of her partner’s serious mental ill health.

“It is difficult to reconcile the weak outcome of this investigation given the complacent response to a series of emergency calls. It is vital that the inquest scrutinises the actions and inactions of mental health services and the police, and whether Suzanne’s death should have been prevented.”

Jan Neate said: “Two families’ lives are totally ruined by what has happened – a beautiful girl who we loved has lost her life and our family life is gone for ever. We are heartbroken. All we have left is to continue fighting to prevent these tragic circumstances from ever happening again.”

Jake Neate has been given an indefinite hospital order. The Essex partnership university NHS foundation trust, which was responsible for Neate’s care, is investigating the case. A domestic homicide review and an inquiry by NHS England are also under way. An inquest into Brown’s death will be held next March.

Essex police declined to comment on the IOPC findings.