Scotland's police watchdog has attacked senior officers for a catalogue of failings following the suspicious death of a young man which was initially ruled a suicide.

A damning report obtained by the Sunday Herald upheld a dozen complaints by the family of Colin Marr, who died from a single stab wound to the chest in 2007 after an argument with his fiancée Candice Bonar at a house they shared in Lochgelly, Fife.

In one of the most critical verdicts ever handed down by the Police Investigations & Review Commissioner, officers were lambasted for destroying DNA evidence against protocol, failing to carry out an internal investigation into the force's shortcomings after Marr's death, and not meeting with Marr's family to take statements until after his body was cremated.

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Leader of the Scottish Liberal Democrats, Willie Rennie MSP, who represents Mid Scotland and Fife, said "unprofessional conduct" by police "prevented the truth emerging".

Bonar told detectives at the scene that the 23-year-old stabbed himself because she had ended their seven-year relationship over an alleged infidelity, which he denied.

Officers accepted her version of events and the case was closed. A subsequent nine-day Fatal Accident Inquiry was unable to determine who inflicted the fatal wound, leaving Marr's loved ones in limbo. Bonar later emigrated to Australia where she now lives with her new partner, Bradley, and their young daughter.

But Colin Marr's mother and stepfather, Margaret and Stuart Graham, have always refused to accept Bonar's story and their campaign for justice led to the Crown Office ordering a review led by retired Strathclyde Police Detective Superintendent David Swindle in 2011. He ruled that Marr's death was "suspicious" and said it "should have been treated as such at the beginning".

A year-long review of the way Fife Constabulary handled the death of Colin Marr and complaints against the force by his family is to be published tomorrow by the Police Investigations & Review Commissioner, John McNeil.

McNeil found senior officers should have considered an internal investigation into the way the death was dealt with as early as February 2008, when the procurator fiscal wrote to a detective superintendent raising concerns.

The commissioner's report said: "The clear and unambiguous position adopted in the Area Procurator Fiscal's letter was that Mr A's death should have been treated as homicide until suicide could be eliminated.

"In the commissioner's view, given this, and the information that was then known, Fife Constabulary ought to have considered at that stage whether all that could have been done had been, and whether a review or internal investigation was necessary.

"Had such a review taken place, it is possible that the serious failings in the initial handling of the incident would have been identified earlier."

Despite these concerns raised by the fiscal, Fife Constabulary broke protocol and destroyed DNA samples provided by Candice Bonar when she was interviewed under caution in May 2008, almost a year after Marr died.

When the Grahams complained, the deputy chief constable told them that, under the Criminal Justice (Scotland) Act 2003, "samples given voluntarily will not be added to the database unless requested by the donor". The commissioner found the act "has no provision to this effect".

His report added: "Indeed, the commissioner cannot find within the 2003 Act the passage quoted by the deputy chief constable in his response."The commissioner ruled that the decision to destroy the DNA samples is "not in accordance with procedure" and ordered Police Scotland to investigate.

Fife Constabulary also came in for criticism because officers did not meet Margaret Graham until after her son was cremated. The family insist the funeral could have been delayed and further inquiries carried out if they had given statements sooner.

The commissioner found that the deputy chief constable at the time took an "overly narrow approach" to the complaint and instructed Police Scotland to issue and apology.

When the meeting did take place, the constable did not "effectively capture" Colin Marr and Candice Bonar's "stormy" relationship. The commissioner recognised that a subsequent investigation by Fife Constabulary acknowledged this "neglect of duty" but McNeill criticised the then deputy chief constable for failing to apologise at the time. The commissioner also found that the force failed to probe whether there was a conflict of interest during the re-investigation of Marr's death. His mother and stepfather complained that a chief superintendent tasked with reviewing evidence assured them he was independent of the initial inquiry but in fact he had been in charge of detectives probing the death just weeks before the incident. The initial complaint will now be reconsidered.

The Grahams also complained that a forensic pathologist instructed in the case had not been provided with statements made by ambulance technicians and by the dead man's colleague. The deputy chief constable was criticised for not examining whether enough information was provided to the professor and Fife Constabulary has been ordered to look again at the complaint.

Pathologists involved in the re-investigation are also to be questioned about an allegation that the police attempted to "unduly influence" them following a complaint by the Grahams that "some time" was spent with the forensic staff before handing over evidence. The report said the commissioner "does not consider sufficient inquiries were carried out into the complaint".

The commissioner also found that a detective inspector involved with the re-investigation made comments to a third party that the family of Marr had been "obstructive during the inquiry". Fife Constabulary has been told to write to the Grahams to apologise for the remarks.

Complaints were also made by the family about the 2011 Fatal Accident Inquiry. The commissioner found Fife Constabulary should have upheld a complaint that a detective inspector "misrepresented" a statement by a friend of Marr about drug use. While under cross-examination, the police officer made "inaccurate" remarks about the extent of Marr's drug use.

The same detective inspector told the FAI that Candice Bonar was not holding a mobile phone when she left the house after Marr was fatally injured, despite several eyewitnesses providing statements that a phone was in her hand. The commissioner said the complaint by the Grahams about this error should have been upheld by Fife Constabulary and ordered Police Scotland to apologise.

The LibDems' Willie Rennie has been supportive of the Grahams since their campaign began. He said: "Seven years after Colin Marr's death a further catalogue of flaws has been uncovered.

"This is on top of all the faults in the investigation for which the Police and Crown Office have already apologised to Colin's family. Now the PIRC want five further apologies, two complaints to be upheld and numerous other actions.

"The police have insisted for years that apart from a limited number of issues with the initial investigation they had made no further errors. This proves that was untrue and must bring an end to the often casual and dismissive approach the police have taken to this case.

"Colin's family have endured years of unprofessional conduct by the authorities which has prevented the truth emerging and compounded the grief. That must now change."

The Sunday Herald contacted Candice Bonar for comment but she did not respond.