Two Canadian experts are disputing the reliability of DNA evidence being used to convict Mark Grant in the slaying of Winnipeg teenager Candace Derksen nearly 30 years ago.

Grant is appealing his second-degree murder conviction in the death of Derksen, 13, who disappeared in November 1984 while on her way home from school.

Grant was convicted following a five-week trial last year that focused on DNA evidence.

But in affidavits filed with the Manitoba Court of Appeal last month, Dr. John Waye and Pamela Newall say there were "undocumented manipulations" to the DNA results, which came from a private laboratory that Winnipeg police hired in 2006 as part of its investigation into Derksen's death.

Waye is a professor of pathology and molecular medicine at McMaster University in Hamilton, Ont., while Newell was most recently a research head at the Centre for Forensic Sciences in Toronto.

Both are agreeing with an American expert who also noted "undocumented manipulations" that all three claim call the reliability of the DNA evidence into question

All three experts also claim the new pieces of evidence had not been given to Grant's lawyer.

Grant is currently serving a life sentence in a maximum-security federal prison in Kingston, Ont.

A date to hear his appeal has not yet been set.

Case was a mystery for years

Derksen's frozen body was found on Jan. 13, 1985 — six weeks after she went missing — on the dirt floor of a rarely used supply shed in a brickyard about 500 metres from her family's East Kildonan home.

Her arms and legs were bound with rope and she was partially wrapped in blankets.

Derksen's death remained a public mystery until May 2007, when police came forward with new forensic evidence linking Grant to the murder scene.

Cold-case investigators charged with solving the homicide had sent hair samples and the twine that was used to tie up Derksen to a lab in Thunder Bay, Ont., that had the ability to run DNA tests unavailable to police at the time.

The results revealed a probability of 1 in 50 million that the DNA profile the lab uncovered from the forensic materials belonged to someone other than Grant.

But at Grant's trial, his lawyers argued the DNA results were unreliable and not to be trusted. An expert witness called by the defence during the trial contended the lab's tests were unsound.