The popular 21-year-old showjumper died six days after being fatally injured in early August 2020.
Her killer, Jonathan Creswell, died by suicide in April after the first day of his trial at Coleraine Crown Court for the murder and rape of Katie.
The ombudsman’s office began its investigation amid serious questions over the decisions made by the PSNI in the aftermath of her death.
Her death was initially ruled a suicide, in large part based on the word of Creswell, who claimed to have found her in the house they shared in Lettershandoney, Co Derry.
Two days after her death on August 9, 2020, it was decided to shut down any further investigation.
This was despite concerns raised by several individuals, including a member of the medical staff at Altnagelvin Hospital.
Creswell’s violent past, including a jail sentence for abusing a former partner, appeared either not to have been flagged or was not considered ahead of the investigation being shut down.
The murder probe was restarted months later, with Creswell finally charged in March the following year.
It is understood a total of 18 officers, ranging from constable to chief inspector, were looked at by investigators from the ombudsman’s office.
Disciplinary action was recommended against six, though two have since retired while it was decided no action be taken against a third.
Action was taken against three officers.However, none of those three are of a senior rank.
Ms Simpson’s family, represented by KRW Law, continue to have questions on the investigation into her death and will be making a response following the publication of the report.
“We have had a circa 1,400 page report from the ombudsman which has identified officers having committed misconduct, not gross misconduct or criminal,” Chief Constable Jon Boutcher told a Policing Board meeting earlier this year.
“That misconduct process is now under way through our professional standards department under the stewardship of the deputy chief constable.”
He added: “Anything we need to do in this organisation with regard to violence against women and girls, we will.
“We now know from Creswell’s lifestyle that there were additional violent offences that he committed.
“Whatever we need to learn from this, we most certainly will learn from this.”
The report is due to be published on Tuesday, November 12.
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