A special garda unit which has been investigating the unsolved murder of Dublin teenager Raonaid Murray has identified a litany of mistakes and oversights in the original probe, the Sunday Tribune has learned.

The Garda Serious Crime Review Team, the so-called cold case unit, has finished its preliminary report into the murder of the 17-year-old in 1999 and has made over two dozen recommendations regarding lines of inquiry that should now be undertaken.

Among the recommendations are that a significant number of new searches should take place in and around the Dun Laoghaire area where the murder weapon may have been disposed of.

However, it is feared that because the murder took place almost a decade ago, any evidence would be long destroyed or tainted.

The cold case investigation, which has been ongoing for a year, has also determined that several potential witnesses who came forward with information at the time were never properly followed up.

It is understood that the Garda Siochana Ombudsman Commission is currently investigating claims by a man who contacted Dun Laoghaire garda station the day after the murder saying that he saw a man and a woman acting suspiciously in Silchester Park on the night of the killing close to the scene.

Although a detective took an initial statement it was never followed up and the man has made allegations that officers forged a statement in his name because they lost the original. The man has been in contact with Raonaid Murray's family about his concerns.

Raonaid Murray was stabbed to death in a laneway between Silchester Road and her home at Silchester Park, Glenageary. She received four wounds from a one-and-a-half-inch knife and collapsed just yards from her home. Her body was found by her sister.

The contents of her handbag were untouched. It was established that there was no sexual motive for the killing, which left gardaí convinced that her murder was personal and that the perpetrator probably knew her or had at least encountered her in the past.

It is also understood that a DNA sample found close to the murder scene is being treated as potentially significant by gardaí.

Last December the Sunday Tribune revealed that detectives believe a female may have been responsible for the murder. This theory is still being given credence and gardaí believe that the woman may have been known to Murray and killed her after a personal disagreement because the schoolgirl broke off contact with her.

The woman is in her 30s and had a reputation for violence against women. She left the country a year after the murder and still lives abroad.

Sources familiar with the case say there was tension among individual garda units during the original investigation and in effect there were two different camps which meant that communication was not as effective as it could have been.

The cold case unit has now ruled out several individuals of interest and has made inquiries in the UK and US. Every statement taken at the time of the murder has been rechecked to see if any information was overlooked.

Although the preliminary investigation has been completed and a report furnished to senior gardaí in Dun Laoghaire for the recommendations to be implemented, it is likely that the overall investigation will take another year to 18 months to complete.

Gardaí say they are confident of a successful outcome to the case.