By Russell Findlay and Susan Ripoll

The death of Darren Birt is one of more than 1000 cases in Scotland where no-one has been brought to justice.

Police Scotland files list at least 1112 homicides since 1960 as 'unresolved' - meaning the force believes they know who committed the crimes but no convictions have been achieved.

Just 60 of these cases are classed as 'unsolved' - meaning the identity of the perpetrators is unknown.

Darren's murder is considered unresolved. Three men were arrested and charged but the Crown Office dropped the prosecution.

Darren's father Brian Birt accuses the authorities of not doing enough to secure a conviction and lacking transparency in their decision making.

He believes the nature of his son's case meant that it did not receive adequate investigative resources.

Darren's death was reported by STV News in 2002 - but otherwise received little media attention with a total of four brief reports in local newspapers.

In an era before social media, there were no online pleas and tributes.

Brian believes this media and online vacuum contributed towards the allocation of resources to his son's case and is frustrated at the police's lack of public appeals in the following years.

His view is supported by David Moran, a former detective inspector with Police Scotland who worked on up to 100 murder cases during his career.

He said: "A case will attract media attention because it is of particular concern to the public.

"Once the media puts it out there the profile has risen and public fears are exacerbated. The police will put more resources into that sort of case.

"Something like a child murder or a female who's abducted, cases of that nature will naturally attract media attention and both the categorisation of the murder inquiry and the amount of resources put into it will definitely be greater than a murder seen as straightforward to solve and of less public interest."

Scotland's homicide rate has seen a downward trend over the past two decades.

In 1998 there were 97, which rose to 119 the following year and in 2002 - the year Darren was killed - there were 127.

Only 2004/05 saw a greater number with 137 victims.

The most recent full year available - 2017/18 - saw 59 victims across Scotland.

According to Moran, the high number in 2002 inevitably put pressure on police time and budgets.

He said: "The resources required throughout Scotland, but particularly in Strathclyde where this murder happened and where the vast majority of these murders would have taken place, it really was a firefighting exercise at times with that volume.

"The murders were generally run by divisional CID, particularly the category C murders, and at times it was just one murder after another.

"The general work the CID would deal with on a day-to-day basis - attempted murders, serious assaults, housebreakings, frauds - that was all pushed aside when these murders happened.

"Where I worked in the north of Glasgow, we found ourselves at times inundated. It was a matter of let's get this one cleared up, we have a sufficiency of evidence, and move on to the next one.

"Did that mean that sometimes there was work that wasn't done as thoroughly as it could have been? Possibly.

"I'm not saying that was a regular occurrence but, yes, I would concede that perhaps some things could have got missed in that."

The veteran officer also believes the creation of dedicated Major Investigation Teams (MITs) and closer police liaison with Crown Office prosecutors has improved standards.

He said: "You had less people investigating many more murders. We're now down in the 50s for murders in the whole of Scotland.

"In the current day you have dedicated MITs which will deal with the majority of murders.

"There's a much more detailed and forensic approach. I'm not saying the murders in the past were incompetently dealt with, but things have moved on.

"Techniques and processes have moved on. You have the Crown with a well-organised homicide unit and taking much more involvement in the day-to-day running and direction of these inquiries and whether they think the police have done enough."

One rarely acknowledged issue which may not sit comfortably with the public, or with families like Darren's, is the perception that organised crime homicides can be allocated greater resources than others.

However, Moran believes this is necessary because witnesses in such cases are often less inclined to speak to the police - and convictions can help stem reprisal killings.

He said: "By the very nature of the people involved in an organised criminal group murder, you will find it a lot harder to gain evidence to prosecute and convict anyone than you would in a murder of less profile or that's happened in straightforward circumstances.

"It's also the likelihood of more violence carrying on from a murder involving an organised criminal grouping.

"You need to try and put a stop to the criminal groups responsible for that violence, otherwise you end up with a spate of shootings, murders like we have currently going on in the west of Scotland."

Both Police Scotland and the Crown Office declined our request for an interview about Darren's case.

A police spokesman said: "We work closely with the Crown Office and meet regularly to review outstanding unresolved murders from across the country.

"Working collaboratively as the Scottish homicide governance group, the potential for new investigative opportunities are regularly assessed in an attempt to review these cases and pursue resolution."

A Crown Office spokesman said: "We treat all allegations of homicide extremely seriously and prosecute where there is sufficient evidence and it is in the public interest to do so.

"The cold case unit works closely with Police Scotland to review cases to ascertain if there are any new evidential developments, including advances in forensic techniques, which would assist in providing a basis for criminal proceedings.

"We recognise the anguish suffered by families who have been denied justice for many years. The family in this case will continue to be updated in relation to any significant developments."

Every homicide in the UK is assigned a category - A+, A, B or C - based on Home Office guidelines.

Various factors, including levels of public concern, public risk and the perceived difficulty of the enquiry, "can be used as a guide to the initial deployment of resources".

The Home Office guidelines state: "Chief officers must ensure adequate financial provision is made for each investigation and, where possible, a suitable work environment is provided for officers working on the investigation.

"They should also regularly review the resources allocated."

Between April 2013 and November 2018, Police Scotland recorded 277 homicides.

STV News found the majority of those - 212 - were category C, which is: "A homicide or other major investigation where the identity of the offender(s) is apparent from the outset and the investigation or securing of evidence can be achieved easily."

Not a single case has been categorised as A+ in that time, which is reserved for cases where public concern is such that normal resource levels would be inadequate.

However, there have been 20 category A cases which are "of grave public concern or where vulnerable members of the public are at risk, where the identity of the offender(s) is not apparent, or the investigation and the securing of evidence requires significant resource allocation".

Additionally, there were 45 category B homicides. These are ones in which "the identity of the offender(s) is not apparent, the continued risk to the public is low and the investigation or securing of evidence can be achieved within normal force resourcing arrangements".

Little is known about the categorisation of homicides in Scotland as police rarely discuss the subject.

However, one of the 20 category A murders since 2013 was that of Irish student Karen Buckley, 24, who was killed in Glasgow in 2015.

Alexander Pacteau is serving at least 23 years for her murder.

The 1998 murder of Lanarkshire waiter Surjit Singh Chhokar, 32, was treated as category C.

Giving evidence into an enquiry about the murder, a senior police officer said that "a category C murder is ... one where the identity of the offenders is known at an early stage".

He added: "A category A murder is one of grave public concern, for example, the murder of a child or a politician. A category B murder is where the identity of the offender is not readily known."

There was criticism of the Crown after two prosecutions in 1999 and 2000 for the Chhokar murder resulted in acquittal.

A change to double jeopardy laws eventually led to the conviction of one man in 2016.