June and Hugh Mcleod believe both organisations ignored a pathologist's findings that their son Kevin suffered massive internal injuries consistent with being severely kicked. The 24-year-old's body was found on February 9, 1997, in Wick harbour following a night out.
"We, the parents of the late Kevin Mcleod ... have devoted all our energies over the last 16 years to ensuring justice prevails for our beloved son, which has drained us physically, financially, mentally and emotionally," they wrote.
"Today we accuse both the chain of command of the now disbanded Northern Constabulary, along with Crown Office officials, of obstructing the course of justice, by ignoring the pathologist who, on Monday, February 10, 1997, alerted the procurator-fiscal of her findings.
"She concluded that the massive internal injuries Kevin sustained were consistent with a kicking received several hours prior to his immersion in the water of the harbour and, that the severity of the injuries was such that, if untreated, they would themselves inevitably have proved fatal."
In 2002, the investigation was reviewed by Andrew Cameron, chief constable of Central Scotland Police, who found shortcomings.
Northern Constabulary was later criticised by Jim Martin, the Police Complaints Commissioner for Scotland, for its "institutional arrogance" in handling the family's complaints.
A Crown Office spokesman said: "The Sheriff issued his determination on 3 December 1998. Should any new evidence be brought to the attention of the Procurator Fiscal, this will be fully considered and the appropriate action taken."
Police Scotland could not comment on the case yesterday.