One volatile mate can land his friends in trouble with police
The message to young people in Britain today is both stark and ominous.
If you are hanging around with friends and someone turns violent and someone else gets hurt, you too could be facing a criminal prosecution.
If someone dies, you too can be charged with murder, even if you did not so much as throw a punch in a melee on the street.
"Standing by is not a defence," is the blunt assessment from the Metropolitan Police's Detective Superintendent Simon Morgan on situations where a gang - or even just an informal gathering of friends - leads to trouble.
Employing a 300-year-old law called Joint Enterprise, police in London are aggressively pursuing young people who are present for violent attacks by their mates.
"Anybody and everybody that is involved in an incident of violence, we will look to identify them and if the evidence is there, we will look to prosecute them," Det Supt Morgan told BBC's Panorama.
Battered to death
Joint Enterprise is about sharing the responsibility of a crime and it ensures that gang members who egg on a friend to violence or who issue a rallying cry to others face the same charges as the person who lands the fatal blow.
The Joint Enterprise route to jail
Tyrone Clark was just 16 when he was set upon and battered to death in the Leeds suburb of Beeston in April 2004 by a gang of local youths.
Four young men who were part of the attack were convicted of Tyrone's murder, even though he died from a single knife wound and the knifeman is still at large.
Lorraine Fraser, Tyrone's mother, said the convictions of the four young men - who ranged in age from 17 to 22 at the time of the murder - are justified.
"They killed my son. They're all involved and they deserve to go down for murder and that's why I welcome the joint enterprise because it sends a message out there."
Mrs Fraser said it is key that young people understand the risks they take in running with a gang.
"On our streets in England that you go in a gang, whatever part you take, and that person dies, you're going down for murder."
It is a theme that the Metropolitan police are taking directly to young people in London via a speaking tour and a video presentation at schools and youth groups.
Kaspa Alexander is serving a life sentence for murder
In the video, Det Supt Morgan does not mince words: "If you are involved in a murder in any way, shape or form, we will come to you. We will find you. We will come at a time when you don't expect us and we will enter your life.
"We will invade your home. Invariably your front door will be removed. We will enter, this will be in front of your parents and your family, possibly your friends, and we will change your life."
Kenneth Alexander, known as Kaspa, is a convicted murderer for his part in the April 2005 stabbing death of Michael Campbell.
What began as small time school ground bullying escalated and led to an open fight in the street between two different groups of friends.
Speaking to Panorama's Richard Bilton, Kaspa explained his emotions at being sentenced, along with four other defendants, to life in prison for his role in the murder.
"I sit here everyday and think the same thing. Am I a con? Am I an inmate? Am I an offender? Have I committed any crime? I walk down the landing
and think am I responsible for taking someone's life? Honest answer - I dunno.
"Some days I think, 'no, I shouldn't be here, I haven't done anything wrong'. I look at that and I think to myself, 'someone's gone'.
It was Kaspa's role in ringing friends to call in reinforcements for a possible confrontation that provided the prosecution with his joint enterprise. That he knew some of his mates carried knives, even though he never did, was also a factor in his conviction.
The application of the joint enterprise law has drawn criticism from some in legal and policing circles who worry that it is being too widely applied.
Among them is Lord Phillips, the country's most senior judge, who has gone on the record as saying that joint enterprise is unfair. The Law Commission has also expressed concern about its use.
Professor Jeremy Horder of the Law Commission said joint enterprise is being used to scoop up anyone who was present at the time, rather than those actually involved.
"It may be that only some members of the gang endorsed or encouraged or helped the killing, others did nothing of the sort.
"But they're all being scooped up in with it."
The Ministry of Justice told Panorama it is looking at joint enterprise, but they are doing so as part of a wider review of homicide laws and the role of those who might have assisted in a murder or a murder conspiracy.
For relatives of victims, it is a necessary piece of legislation that ensures that those who stood by and watched their loved ones being attacked are not deemed blameless.
Among the country's most high-profile murders in recent years was the beating to death of Cheshire father of three Garry Newlove.
Mr Newlove confronted a group of drunken young men in the street outside his home when he suspected they were vandalising his wife's car. He was set upon and punched and kicked to death in front of his family.
Garry Newlove's killers, Adam Swellings (l), Stephen Sorton and Jordan Cunliffe
A trio of young local men were convicted under joint enterprise of his murder, even though the pathologist report said that it was just one fatal kick to the head that killed Mr Newlove.
One of the three convicted young men, Jordan Cunliffe, has maintained his innocence, saying he did not participate in the attack. Both the prosecution and the defence have accepted that Cunliffe, who was 15 at the time of the attack on Mr Newlove, is virtually blind.
He also denies being present at an earlier attack by the gang - his presence at that beating was a key part in the prosecution's case of proving that Cunliffe knew that his friends were capable of severe violence.
His lawyers hope to appeal his conviction and 15 year minimum prison time of his life sentence.
But Helen Newlove, Mr Newlove's widow, said Cunliffe is a murderer and, if anything, his minimum prison term, is too light.
"Would you stand there watching somebody else kicking and punching? Would you actually think that was right to watch even if you didn't do the act? Because I certainly wouldn't. They were as guilty as the person doing the act."
Panorama: Lethal Enterprise, BBC One, Monday, 23 November at 2030GMT.