An inquiry into the handling of the harassment case of a young father before he was murdered has identified "serious flaws in the police response".
Peter Woodhams was first attacked in January 2006
The Independent Police Complaints Commission (IPCC) found officers had not followed up available evidence.
The family of Peter Woodhams, 22, from Newham, east London, claims police failed to protect him from youths who harassed him before his death.
The father-of-one was shot dead outside his home in Canning Town last year.
Mr Woodhams' ordeal began after he confronted youths who threw a stone at his car in January 2006.
He was stabbed and slashed across the face and neck in the assault.
The IPCC's executive summary highlighted failings with the initial response to the stabbing as well as the subsequent investigation.
Officers responding to the stabbing failed to seize evidence for forensic analysis, namely Mr Woodhams' clothes and a blanket which had been wrapped around him following the stabbing, and failed to take a "proper account" of the incident.
Tucker was jailed for Mr Woodhams' murder
They failed to "ensure that appropriate investigative opportunities were maximised", the report said.
Regarding the subsequent investigation, the IPCC said officers' contacts with the victim's family were inadequate.
For example, they failed to take a witness statement from Mr Woodhams' fiancée Jane Bowden.
The report found detectives failed to investigate a list of suspects in the stabbing provided anonymously to the Woodhams' family.
With both the initial response and the subsequent investigation, the report found officers "failed in their performance of duties".
However, the IPCC said the Metropolitan Police had "recognised and responded" to the issues highlighted in the report.
Officers in Newham have since received training on evidence gathering and serious assault cases now come under the supervision of senior management.
Two sergeants and a detective sergeant have already received written warnings for their failings in the first investigation, following the IPCC investigation.
A detective constable has also been given "words of advice". Three more officers were cleared of wrongdoing but will be given guidance.
Two detectives are facing a disciplinary hearing.
In May, 17-year-old Bradley Tucker was jailed for life for murdering Mr Woodhams outside his home.