The Independent Police Complaints Commission will investigate the Metropolitan Police over its handling of the stabbing of Peter Woodhams, who was later murdered near his home in east London, it said yesterday.
The news came as Britain's most senior police officer said he had been asked to report on his force's handling of the case.
Mr Woodhams, a 22-year-old father-of-one, was shot just moments after a confrontation with the gang outside a parade of shops in Canning Town, east London.
Only seven months ago he had been attacked by a group of youths and stabbed in the neck near the same shops.
Scotland Yard Commissioner Sir Ian Blair has been asked to "report fully" to the Metropolitan Police Authority on how his officers responded to the stabbing and Mr Woodhams' subsequent murder.
His fiancee, Jane Bowden, claims she phoned police every day for five weeks following the attack, but that officers never even took a statement from her.
It emerged yesterday that the 14-year-old youth arrested over Mr Woodhams' murder this week had also been held in connection with the earlier stabbing. The teenager, who has been released on police bail, was not only arrested in connection with the murder, but also on suspicion of causing grievous bodily harm in relation to the knife attack on January 21.
Commander Rod Jarman told a press conference: "We will robustly deal with any failings in our investigation of the previous incident." Detectives are examining CCTV of Monday's confrontation outside the shops in Freemasons Road in the hunt for clues.
Shortly after the altercation, Mr Woodhams drove the short distance home to Tallis Close and went in, complaining to his fiancee of "trouble outside".
He went back out, apparently to speak to the youths, but just a few moments later he was shot while just yards away in adjoining Leslie Road.
Mr Woodhams, a television satellite engineer, managed to stagger back towards his house, before collapsing in front of his horrified fiancee.