The body of Kevin Mcleod, 24, was recovered from Wick Harbour in February 1997 after he had been on a night out with friends.
Last year Police Scotland admitted Northern Constabulary had failed to treat the death as murder, despite an instruction from prosecutors.
Mcleod’s parents, June and Hugh, have now written to Dame Elish Angiolini, Scotland’s former chief prosecutor, amid anger at delays in responding to concerns about how their case has been handled.
They also want the former lord advocate to look at whether complaints against senior officers could continue to be investigated even after they leave the force.
Angiolini, who was lord advocate from 2006 to 2011, is carrying out a review of the police complaint framework on behalf of the Scottish Government.
The Mcleods are still waiting on a response from the Scottish Police Authority (SPA) after it was told three months ago to explain why a recent review initiated by former Chief Constable Phil Gormley failed to examine Northern Constabulary’s decision not to treat the case as murder.
In their letter to Angiolini, Mcleod’s parents said: “In order to ensure continued public confidence in the police, consideration should be given to the introduction of provisions whereby officers of any rank who resign or retire before the conclusion of disciplinary proceedings can still be held accountable for their actions.”
The family also called on Angiolini to examine extending the powers of the Police Investigations and Review Commissioner (Pirc).
They said: “The Pirc has a timescale of 56 days for any police bodies operating in Scotland to implement their recommendations. However to date, and after 86 days, the SPA has still failed to provide us with a response or to implement the Pirc recommendation.
“It appears the remit of the Pirc does not have the power to compel the SPA to respond in the timescale set out. Therefore the remit of the Pirc is flawed and clearly not fit for purpose.”
Labour MSP Daniel Johnson said: “More than 20 years after the death of Kevin Mcleod, his family are still looking for answers following a series of complaints that have been mishandled by the police. While their complaints have been upheld, the Pirc don’t have any power to compel the SPA or Police Scotland to even respond to the family in a timeous manner.
“This one example shows that the review due to be carried out by Dame Elish Angiolini is vital in improving the police complaints system, through which so many people have had difficulty navigating and getting the answers they deserve.
A spokesman for the SPA said: “We understand the Mcleod family’s frustration at the time it has taken to implement the Pirc’s recommendation, which has arisen because of the need to make a number of additional enquiries into the complaints. Efforts are being made to complete these enquiries as quickly as possible, and in the meantime we shall continue to liaise with the Mcleod family and the Pirc.”