By Russell Findlay and Susan Ripoll

Darren Birt was 22 when he was brutally attacked by three men armed with hammers and knives.

He died lying in a pool of his own blood in Barlanark in Glasgow's east end. Seventeen years later, no-one has been convicted of his murder.

His father Brian copes with the "indescribable" pain by making a monthly pilgrimage to Calvay Place, the residential street where his son's life was ended.

"They smashed his head in with a hammer and stabbed him in the back multiple times," he said. "They left him lying in his own blood.

"He had everything to live for. What really upsets me, coming here, is I know in my mind's eye that this is where my son took his last breath.

"I was five minutes away in the house watching TV while my son was lying dead in the street."

For Brian, his former wife Anne and their other son Kevin, the suffering is worse because Darren's killers remain free.

The three suspected perpetrators were young, white males. The murder attracted virtually no media coverage despite its violent nature.

Although three men were charged, they were never prosecuted. Brian suspects the police and Crown Office did not invest meaningful time or effort into his son's investigation because of an unspoken 'postcode lottery' for murder inquiries - with the victim's social status being a factor in the allocation of resources.

During his visits to Calvay Place, he recites a pledge to his son that he will not rest until justice is done.

This commitment is now a campaign to highlight what he believes are Scotland's "forgotten murders".

The 62-year-old, who has survived cancer but fears its return, said: "I speak to Darren and say 'I'll never let you down son, I'll keep going and going until I got some sort of justice for you'.

"It makes me feel very sad and very angry because nothing has been done for my son and the murderers are still out there.

"It's a horrendous feeling. It's one that will never go away for me, my son Kevin or my former wife.

"The pain in all our hearts is unbelievable. It's indescribable pain and it's just not right that this can be a forgotten murder."

Looking through photos of Darren as a youngster, Brian said: "You look at all the photographs together of a happy family and one day it's taken away from you.

"You're just an ordinary family and then your son gets murdered."

Brian will never forget the morning when two police officers arrived with the news.

"Kevin was only 13 when the police came to the house and he didn't realise what had been said. Quite honestly, neither did I. I just froze.

"It was a horrendous time - a complete stranger telling you your son's been murdered and that's it, then they walk out the house.

"Me and my wife sat Kevin down and just told him the truth - than his brother had been murdered.

"When Darren died, we all died with him and we're doing a life sentence just now. It's hard to explain the feeling deep inside - it's right in there and it never goes away."

Darren's mum Anne has battled mental health issues and has had numerous spells in hospital since his death.

She said: "When that happened to my son knocked me back. I wouldn't take my medication and I wouldn't eat.

"My nurse used to come see me now and again and I used to say, 'Where have I been?', and she'd say 'Anne you've just had a shock'.

"17 years is a long time to lose your memory and now I'm back, I've got my memory back."

While Brian feels closest to Darren at the murder scene, Anne prefers to visit her son's grave and also prays for justice.

"You have a moment between the two of you and I say a wee prayer and think of all the happy times, that's all you can do.

"He didn't deserve what he got and I wish somebody would come forward and see the pain that we have had."

Brian has written a self-published book titled Scotland's Forgotten Murder, which is critical of Police Scotland and the Crown Office.

He said: "Before my son's murder, I had every faith in the police. But in the 17 years since my son was murdered, I've become totally frustrated with them and the Crown.

"I've tried everything I can to try and get justice but I feel I'm up against a brick wall with the legal system. Everything I ask them they can't tell me, they say it's confidential.

"I've been told on numerous occasions it's not down to money but that's not the case - it is about the money."

Brian hopes the passage of time and changed personal circumstances may prompt witnesses who stayed silent in 2002 to now speak out.

He said: "I've been told by numerous sources that there were people who burnt clothes, hid the hammer, hid the knives. My son's dead and there are three murderers out there, walking free, who have never paid for it.

"How would you feel if it was your son? If you've got anything about you at all, pick up the phone and let the police know.

"I made a solemn promise to Darren that I would get justice for him in my lifetime and I won't stop fighting for justice for my son until my last breath."

Former Police Scotland Detective Inspector David Moran told STV News that the volume of work his department faced in north Glasgow could impact on how cases were dealt with.

He said: "There was one murder coming in after another and it was a matter of 'let's get this one cleared up, we have a sufficiency of evidence'and move onto 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 somethings could have got missed in that."

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

In a statement, 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."