A key piece of evidence needed to review a man's murder conviction isn't missing, RCMP in Manitoba said on Tuesday. It was destroyed years ago.

A provincial review committee wanted to test the DNA of hair that was used to convict Kyle Unger and Timothy Houlahan for the killing and mutilation of 16-year-old Brigitte Grenier in 1990.

The single strand of hair couldn't be found. On Tuesday, RCMP Sgt. Steve Saunders said the single strand of hair had been destroyed as a matter of police policy.

The law requires police to keep evidence for 30 days after all avenues of appeal have been exhausted, Saunders said. After that, police have to weigh the value of storing exhibits such as bloody clothing and hair.

"If you've been at many of our detachments out in the field, they're really small," he said. "And so to retain a lot of exhibits, particularly biological exhibits, there are really strict procedures to maintain the integrity of those exhibits."

Unger's final appeal to the Supreme Court of Canada was rejected in December 1993. Some time after that, police disposed of the hair.

Houlahan killed himself in 1994.

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Recent developments have the Mounties working on new guidelines for preserving evidence.

A report into the wrongful conviction of Thomas Sophonow recommended all physical evidence from murder trials be kept for 20 years.

Sophonow was finally cleared in 2000 of the 1981 murder of Barbara Stoppel. He had been tried three times before DNA evidence exonerated him.

Last fall, James Driskell was released from prison after DNA analysis proved the original forensic analysis of hair evidence had been wrong.

Since then a committee has been reviewing all cases where hair evidence helped prosecutors reach convictions.

The chair of the committee says police forces everywhere need to reconsider their policies on storing old evidence.

"Exhibits that are capable of perhaps later analysis in a realistic sense could be reassessed with a developing science, sure, those kinds of exhibits should be maintained," said Rick Saull.

Before DNA analysis was available to police, hair evidence was analysed by comparing qualities such as colour and texture.