PC Keith Blakelock was stabbed to death during the Broadwater Farm riots in Tottenham, north London.
The 40 year-old father-of-three was attacked by a mob armed with sticks, knives and machetes on October 6, 1985.
PC Blakelock, originally from Sunderland, was one of the first officers called to the estate.
He and his colleague PC Richard Coombes were attacked when they moved in to protect firefighters trying to put out a blaze at the Tangmere block of flats.
The officer suffered a total of 43 stab wounds before being pulled away by colleagues. A knife was left embedded in his neck and an attempt had been made to decapitate him. His police helmet has never been recovered.
Three men, Winston Silcott, Mark Braithwaite and Engin Raghip, were convicted of the murder of PC Blakelock in March 1987. The convictions were quashed in 1991 following allegations of police fabrication of evidence.
A second investigation between 1992 and 1994 under Commander Perry Nove did not result in any charges but identified several suspects (the 'kickers') who were willing to give evidence against the main attackers (the 'stabbers').
The third investigation began in 2000 and involved unsuccessful attempts to retrieve forensic evidence from the exhibits.
In 2010 detectives reinvestigating the case arrested 14 men on suspicion of involvement in the murder. All but one were released with no further action.
On 23 July 2013 police charged Nicholas Jacobs, 44 [13/10/68] of Hackney, with the murder of PC Keith Blakelock.
Jacobs went on trial at the Old Bailey on 3 March 2014. The prosecution case relied on testimony from three eyewitnesses that he was one of those using a knife during the attack on PC Blakelock.
The witnesses gave evidence using the pseudonyms John Brown, Rhodes Levin and Q. Brown and Levin, who had both served jail terms after being convicted of taking part in the riots, admitted they kicked PC Blakelock during the attack. In 1994 Brown received a £5,000 reward and Levin a £2,500 reward for their willingness to give evidence after the Crown Prosecution Service decided there was insufficient evidence to prosecute anyone.
Witness Q, who first came forward in 2009, claimed he was standing around 15ft away.
Other evidence against Jacobs included photographs of him taking part in the riots on a different part of the estate and his conviction for violent disorder in 1986. While in prison a rap lyric was found in his cell which appeared to indicate he was involved in the murder.
It read: 'As long as I live I will remember it was 1985 the 6th October... we gone at Broadwater I have on an Clarks, him have on an booger him have on his jacket I have on an jumper him have his bike helmet I have me balaclava him have hold of 007 (knife) and me have de chopper we have intention to kill an police officer PC Blakelock de unlucky f**ker him dis an help de fireman... who did an out fire de fireman see we av come and decide to scatter but PC Blakelock him never smell the danger but when we fly down upon him he start scream and holla everybody gather round and av pure laughter he try to head out but we trip him over he start beg for mercy but it didn't matter him try to play super man... and him ger capture him have too face the consequences we back out we chopper we start chop him on his hand we chop him on him finger we chop him on him leg we chop him on his shoulder him head him chest him neck we chop him all over when we done kill him off lord er feel much better... me just wipe off me knife and go check on daughter we sit down and talk and she cook me dinner we smoke some sensi and we drink some liquor and then we make sweet love until a little later she suck me buddy... anyway five days later me ger capture de police and they kick off my door like me an mass murderer and they search up me flat like me a drugs smuggler they charge me with affray they have to drop the murder because I keep me mouth shut never turn no informer...'
The court also heard how Jacobs was arrested on suspicion of burglary on 3 May 2000 and told the police officer: 'F*** off, I was one of them who killed Keith Blakelock.'
Jacobs did not give evidence in his defence. His barrister Courtenay Griffiths QC argued the witnesses were unreliable, proven liars who were motivated by reward money.
On 9 April 2014 the jury acquitted Jacobs of murder and manslaughter.
A statement on behalf of PC Blakelock's widow Elizabeth and three sons said: "We are obviously extremely sad and disappointed at the verdict. We viewed this trial as an opportunity to see some form of justice served for Keith.
"There were many people involved in a murder on that night of 6 October 1985 and it is regretful that no one has yet to be found guilty despite the number of people with knowledge of the events of that night.
"We appreciate the work and effort over the years since events on Broadwater Farm that night in trying to bring people to justice.
"We hope that more people are able to come forward so that some of those guilty can be brought to justice in the future."
Assistant Commissioner Mark Rowley, said: "PC Keith Blakelock joined the Met Police to serve and protect the community of London. That desire ended in his murder, following an attack that was without mercy, his murder was barbaric.
"That terrible night in October 1985 Keith, and his colleagues of Serial 502, were sent to the Broadwater Farm Estate to protect fire crews battling to control a blaze that was threatening to take hold of a block of flats. Keith was only present that night to do what he joined to do - protect and serve the community of Tottenham.
"We have worked closely with the Crown Prosecution Service over many years to be in a position to put our evidence before a court. The investigation team pursued all the material, evidence and witnesses they could. No matter how difficult an investigation it has been to carry out - given the many years that have passed, the lack of forensic evidence and CCTV plus the main witnesses to Keith murder's being those taking part in the riot - it was important we exhausted every possible lead we could.
"Sadly, Keith's widow, family and friends still have not seen anyone brought to justice for his murder. The dignity, extraordinary patience and courage they have shown in their nearly thirty year quest for justice is humbling.
"We will not give up on bringing Keith's killers to justice. There are people who know exactly who took part in the attack on Keith and people who took part themselves. It is not too late for you to come forward. Almost thirty years on peoples' lives are very different, their allegiances broken or shifted. Help us now.
"The 6 October 1985 was an extremely sad day in the history of policing, and no police officer serving at that time will ever forget it. Today everyone in the police family should reflect on the barbaric and tragic events Keith, and Serial 502, experienced that night."