The police investigation into the heroin death of Rachel Whitear has been criticised in an official report.
The 21-year-old shop assistant was found dead on the floor of her bedsit in Exmouth, Devon, with a syringe in her hand in May 2000.
A report by Wiltshire Police into the Devon and Cornwall Police investigation found a "general organisational failure" but omissions did not amount to a breach of the Police Code of Conduct by individual officers.
Rachel's parents, Pauline and Mick Holcroft, from Ledbury, Herefordshire, were present at a news conference in Exeter when the Independent Police Complaints Commission report was published.
The couple said Devon and Cornwall Police had not followed their force policy guidelines that death that might be attributable to drugs should be treated as suspicious until proven otherwise.
They said in a statement: "Evidence destroyed, paperwork and witness statements lost, lack of decisions, leaders and lack of direction and too many assumptions by Devon and Cornwall Police has meant the result of all this hard work has effectively got us little further."
A jury at a second inquest in September last year, concluded Ms Whitear died from heroin intoxication, but was unable to say whether she injected herself or was alone when she died.
After the inquest Miss Whitear's parents criticised Devon and Cornwall Police, the coroner and the pathologist involved in the first investigation.
Chief Superintendent Paul Howlett, of Wiltshire police, said today: "While their investigation had been able to provide new information about the circumstances of Miss Whitear's death, I very much regret that despite our best efforts some questions continue to be unanswered."
He said during the investigation it was found that potential lines of inquiry were not pursued.
One involved a missing tobacco tin which potentially indicated someone might have entered Rachel's house before her death.
The other was a change of account by her boyfriend, Luke Fitzgerald, about his last contact with her prior to her death.
"It has been assessed, however, that neither of these omissions amount to a breach of the Police Code of Conduct by individual officers, but do indicate a general organisational failure within Devon and Cornwall Police.
"This is a view that is supported by the Independent Police Complaints Commission," Mr Howlett said.
"It is evident that officers who attended the death scene did not believe it to be suspicious and there were valid reasons to support that conclusion.
"On this basis it seems to have been concluded by those concerned there was no immediate apparent reason why Rachel's death should have been treated as suspicious and therefore no reason why a Home Office pathologist should have been requested to attend the scene in compliance with force policy," he said.
The decision not to complete a post mortem on Rachel's body was made following discussion between the coroner of that time and a local hospital pathologist, Mr Howlett said.
He added that the decision fell outside his terms of reference.
"It seemed the detective constable given responsibility for conducting the death investigation was not aware of that decision and that he expected a post mortem would be carried out and that he would receive a pathologist's report.
"It seemed his superiors were aware there was to be no post mortem," Mr Howlett said, adding: "In my view it would not have been unreasonable for them to have discussed the decision with the coroner to avoid any potential ambiguity.
"Additional toxicological tests at that time could have provided the first inquest with sufficient evidence to provide a cause of death as was the case at the second inquest.
"In general terms it seems to me that the detective constable would have benefited from greater support and intrusive supervision from his supervisors," Mr Howlett added.
The first inquest in December 2000 recorded an open verdict with no cause of death established after no post mortem was held.
After Mrs Holcroft complained to Devon and Cornwall Police about the investigation, Wiltshire Police were appointed to reinvestigate the death.
Miss Whitear's body was exhumed from Withington churchyard in Hereford in 2004 for a post mortem and further tests.
And the High Court ordered a second inquest.
Luke Fitzgerald, from whom Miss Whitear had split the day before she died, told the second inquest he was not present at the time of her death.
He and his brother Simon were arrested on suspicion of being involved in Rachel's death, and a later bid to tamper with the death scene.
But the Crown Prosecution Service concluded there was insufficient evidence to justify criminal charges.