Karl Bishop stabbed Rob Knox five times
A man who stabbed to death 18-year-old actor Rob Knox had already been under investigation for a knife-point burglary, police have said.
Karl Bishop, 22, has been found guilty guilty murdering the Harry Potter actor outside a Bar in Sidcup, south-east London in May 2008.
But he was under investigation for robbing a man two months earlier.
Although there was not enough evidence to bring charges, two officers had been disciplined after an internal inquiry.
Two weeks after the robbery the victim's mother reported a burglary at his flat in Chislehurst, Kent, and named Bishop as the suspect.
Officers were still investigating the robbery claim when Bishop, armed with two kitchen knives, confronted Rob outside the Metro Bar and stabbed him five times.
When the officers concluded their inquiry several weeks later, prosecutors said there was insufficient evidence to take it to court.
Officers investigating the burglary also could not find enough forensic or other evidence to support a prosecution.
But the Metropolitan Police (Met) said two officers involved in the robbery inquiry had been given written warnings as a result of the internal investigation.
Rob Knox (right) had a small part in the latest Harry Potter film
A Met spokesman said senior officers have also set up a new system to improve the way police monitor suspects for all violent crimes.
He added: "The Met realised the existence of this earlier allegation to police concerning Karl Bishop would concern the family and the public.
"The Met acted promptly to address this by asking its Directorate of Professional Standards (DPS) to investigate what had happened.
"The Met also voluntarily referred the matter to the Independent Police Complaints Commission (IPCC), which supervised the investigation into what happened.
"A thorough investigation was carried out and as a result two officers involved in the robbery investigation were given written warnings.
"The Knox family was kept informed of this DPS investigation throughout and of all measures taken as a result of its findings."