Prior to Cresswell’s arrest, several people, including relatives of Ms Simpson, had raised concerns about him.
He had a previous conviction for an attack on a woman in 2010.
The ombudsman’s investigation, which began in March 2021, resulted in a file of 1,400 pages being sent to the PSNI.
It found misconduct by a number of officers, with their cases dealt with by the PSNI’s professional standards unit.
The investigation followed a number of complaints lodged by Ms Simpson’s family, related to the fact that police did not treat her death as murder at an earlier stage.
They are bringing a civil action against the PSNI.
It is understood a number of further complaints related to the case have been made to the ombudsman more recently.
They are now under investigation.
