According to its commissioner, a 12-year-old boy died when a man with schizophrenia drove into schoolchildren.
Harley Watson was struck outside Debden Park High School, Loughton (Essex), on 2/12/2019
Terence Glover (52), was held indefinitely last year under the Mental Health Act.
Roger Hirst (police, fire, and crime commissioner for Essex) said that "much more could have and should be done".
An inquest determined that Harley had been unlawfully killed on Thursday.
According to the jury, his death was possibly caused by failings of Essex Police and mental healthcare services.
In Chelmsford, Glover was a paranoid schizophrenia patient who made repeated 999 calls before the crash. He said that he might "run some schoolchildren over."
In September 2019, he was arrested on suspicion of making malign communications. However, a mental assessment that lasted less then three minutes revealed that he was not in custody, according to the inquest.
Glover was not properly evaluated by the jury and Harley's death was prevented.
Jurors found that police did not invest enough resources in the case and they didn't pass concerns about Glover between departments. Glover was released from custody because of this failure.
Mr Hirst stated: "It is evident to me from both the evidence presented throughout the inquest, and the jury's narrative conclusion, that much more could have and should have been done before the incident."
"We all work hard in the emergency services to protect people's lives and make them better. But, in this instance, we collectively let Harley down.
"I truly regret that."
Hirst stated that the chief constable of the force had "assured him" that "significant improvements had already been made".
He stated that it was vital for the force to improve their ability to identify, assess and respond to threats, particularly when they relate to cases in which mental health may be a factor.
Andy Prophet, Deputy Chief Constable, apologized for the "failings of Essex Police" he identified after the inquest.
He said, "We have provided more training to our call handlers and to our colleagues who respond on the calls we get for help,"
Essex County Council has an adult mental team and a spokeswoman said that it had apologized to Harley's family for the "flaws in the Mental Health Act assessment" performed on Mr Glover 30 September 2019.
According to the authority, it has since improved its operating procedures as well as implemented safer practices.
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