The West Mercia police force was guilty of "a number of failings" in investigating a rape claim made by a man who was later killed by the person he accused, a watchdog has ruled.
The Independent Police Complaints Commission (IPCC) found officers within West Mercia Police did not take the allegations by Adrian Palmer seriously enough and left the investigation in the hands of a junior constable.
Mr Palmer, 21, reported an allegation of rape to police in January 2006, and followed this up with complaints that he was receiving threatening texts and voice messages on his mobile phone.
The IPCC found the "relatively junior" constable handling the allegation was given minimal supervision, "was not fully aware" of the force's policy on sex offences, and did not even know of the existence of specially trained sexual offences liaison officers.
The police force missed the opportunity to gather potential evidence by failing to give Mr Palmer a medical examination. Police also failed to seize Mr Palmer's phone for forensic analysis, meaning the identity of a caller who left threatening voice messages was never discovered.
Some incidents reported in person at Tenbury Wells police station had not even been recorded and after the eventual file was handed to the Crown Prosecution Service incomplete, it was ruled in March 2006 there was not enough evidence to proceed.
Mr Palmer, who suffered from Asperger's Syndrome, was found dead in a car park in Tenbury Wells on May 20. The local man he had accused of the rape, Ben Murphy, was found guilty of manslaughter in December 2007 and sentenced to four years in prison.
IPCC Commissioner, Len Jackson, said: "He deserved to have his allegation dealt with more seriously by West Mercia police. We found a number of failings by individual officers who were left in charge of the inquiry, and certain lapses by them undoubtedly undermined the criminal investigation.
"However ultimately we cannot say that even had the rape investigation and reports of threatening mobile phone messages been looked into correctly, Adrian Palmer would not have been killed on 20 May 2006."
West Mercia Police said all the recommendations made by the IPCC had been "accepted and implemented".
A constable and a sergeant "will be issued with words of advice in the near future" by Assistant Chief Constable Simon Chesterman.