Police admit their evidence-handling system is a shambles, with crucial exhibits from hundreds of unsolved crimes going missing for years, including items from the Hayley Dodd murder mystery.
A special task force has been working for almost a year trying to unravel the mess, examining the contents of evidence crates stored at a secret facility in Perth's eastern suburbs.
As part of that search, some exhibits are being sent for urgent retesting in the hope that advances in forensic technology can help solve some of the State's most troubling crimes.
Assistant Commissioner State Crime Craig Ward confirmed yesterday that the crimes included murders, rapes, robberies and other violent offences, some dating back to the 1980s.
"There are a whole bunch of crates out there and we are not confident that what is marked on the front of the crate is actually what is inside," he said.
Mr Ward admitted that as a result of the confusion, it was a concern to police that some criminals who could have been caught years ago were left free to keep offending.
"It is an absolute concern . . . I think we've seen in other jurisdictions where those things have occurred and we are not arrogant enough to think it wouldn't happen here," he said. "There has been chances gone begging, I would suggest, because we haven't known what was in boxes or if we had, we hadn't dealt with them appropriately in some cases."
A key piece of evidence from the Hayley Dodd investigation is understood to have been missing for more than six years before it was finally found again.
Not long after that discovery, a new inquiry was launched that included a big search at a Badgingarra property not far from where 17-year-old Hayley vanished on July 29, 1999.
The former owner of that property - Francis John Wark - is in jail in Queensland for abducting and brutally raping a young woman in 2007.
Police have asked _The Weekend West _not to reveal what the missing piece of evidence is, only that it is a personal item believed to belong to Hayley.
The original team of investigators who worked on her disappearance found it inside a car that Wark was allegedly driving on the day she disappeared.
DNA testing was in its infancy at the time and police could not conclusively prove it belonged to Hayley, though it is believed the item closely resembled a description provided by one of her friends.
Police have refused to say if they have now been able to link it conclusively to Hayley using updated testing techniques.
Many of the problems with evidence handling began when a new centralised storage facility opened in 2006.
Because of the sheer volume of material that flooded in, crates were simply stacked on shelves without being inspected or their contents catalogued.
"It is likely there is going to be some criticism of past practices but we have to be upfront and if we can solve some serious crimes and give some comfort to victims and secondary victims of some of the worst crimes in the State, that has to be a great thing," Mr Ward said.