A couple targeted by a crossbow attack which left one of them dead and the other seriously injured had reported the suspect to the police a year before he struck, the Independent Office for Police Conduct (IOPC) has said.
Shane Gilmer, 30, was murdered at his home in Southburn near Driffield, Humberside, on January 12 and his partner Laura Sugden was also injured.
Their neighbour Anthony Howarth, also known as Anthony Lawrence, was the suspect who fled and the 55-year-old was found dead in a car in North Yorkshire the next day.
The IOPC said the couple had made "a number of reports" about Howarth, dating back to January 2017.
The watchdog will investigate Humberside Police's response to their complaints and will look at the nature of the contact with police, how this was recorded and how the force responded.
The IOPC said the manhunt for Howarth and his death were not part of the investigation.
An inquest into Mr Gilmer's murder was due to be held later.
IOPC regional director for the north east, Miranda Biddle, said: "We carefully considered the referral from Humberside Police before beginning our investigation, making the decision to look at the reports by Mr Gilmer and Ms Sugden and what, if any, contact the force had with Anthony Howarth dating back to January 2017.
"We have met with Mr Gilmer's family and with Ms Sugden, to discuss our investigation, and they will be regularly updated on its progress.
"I am confident that our investigation, and of course the coronial proceedings to come, will bring some clarity to the circumstances of this horrific murder and attempted murder."
The IOPC said Mr Gilmer's family had asked that the media respect their privacy.