The trial and conviction of two men for the murder of 16-year-old Agnes Sina-Inakoju, shot while waiting in a takeaway pizza shop, has raised questions about the extent of gang culture in British cities. But how much do we know about gangs and how they operate?
What do we know about gangs in the UK?
There is an awful lot of anecdotal evidence - but very few hard facts. Criminologists have researched gangs for decades - but part of the problem is that the word "gang" itself can be misleading. Gangs are not new - they've often been fundamental to youth criminal culture. Both Charles Dickens and Graham Greene wrote about gangs.
So what do we mean by the word "gang"?
From a policing perspective, a gang usually means a group of people involved in crime. But a gang can also mean a group that provides emotional or psychological support to its members. These two definitions are not exclusive.
Then there are other factors. Some gangs are territorial - such as those involved in selling drugs. Others are based on ethnicity. The latest factor in how some gangs organise is social media, which plays an increasingly important role in the life of youngsters.
The Youth Justice Board (YJB) said the word should not be over-used because it can glamorise what would otherwise be relatively minor anti-social behaviour.
This appears to be particularly important in relation to teenagers on the fringes of offending who are seduced by some of the symbolism and belonging that comes with gang culture.
In the past, the Home Office has broken down the definition of gangs into "three tiers of risk", which describe a "conveyor belt" from minor teenage misdemeanours to serious crime.
But gangs do play a role in youth offending?
Most criminologists say young people who break the law, generally do so in a group environment - if you are emboldened by friends, it becomes easier to think you can get away with it.
Three of the UK's most important criminal justice watchdogs warned in 2010 that efforts to tackle teenage gangs were missing important opportunities to rescue youngsters from criminality.
The joint report by the chief inspectors of prisons, police and probation said strategies were patchy and often counter-productive.
Like the Youth Justice Board, they cautioned against exaggerating the power of gangs - but warned schools and prisons against ignoring them.
What are the key crimes relating to gangs?
Drugs and all the crimes associated to drug-pushing. Professor John Pitts, of the University of Bedfordshire, studies gang membership. He has found the fundamental change in gang culture had been the development of the drugs market. Drugs are a major business that requires an "expanding workforce" to keep the goods moving to addicts.
And the gang fulfils that role?
Younger gang members protect a geographic patch, ensuring efficient delivery of the drug itself and protecting the senior figures from detection. The street gang, in the words of one leading sociologist, becomes the "shop floor of the international drugs business".
Are these teenagers all willing participants?
Professor Pitts found in some of his research that 40% of the young people he came across were "reluctant gangsters" - teens with no criminal record. Some of these kids were scared of violent reprisals if they left a gang or disagreed with its aims. In that context, affiliation became a form of reluctant protection for their family.
How many gangs do we think exist?
It's very difficult to say and there is no single official number. Police in London and Strathclyde have identified 170 gangs respectively, while officers in Liverpool and Manchester have said in the past that 60% of shootings in the cities are linked to gangs.
One 2006 study for the Home Office found that between 6% and 10% of 10- to 19-year-olds said they were in a gang.
In short, experts say it is very difficult to put a number on these things because groups constantly change, rename themselves, reform or break up. What is perhaps more important is to look at who is affected.
Two thirds of Youth Offending Teams - the local units tasked with rehabilitating young offenders - told the YJB they knew of "troublesome youth groups".
Professor Pitts's research suggests that the activities of a hardcore of 40 gang members running a serious crime enterprise can ultimately affect the lives of 6,000 people.
Where do weapons come into the equation?
Again it's very difficult to say. A Home Office study published in 2004 looked at arrested adults who declared gang membership.
It found an increase in the numbers claiming to carry any weapon, but little change in the number claiming to have carried a gun. Experts think the increase is down to more knives being in circulation.
What drives membership?
Gangs develop in specific local circumstances - but key factors appear to be family problems leading to dependence on friends, and school problems such as exclusion.
The gang environment, say some experts, can provide the "respect" that the members crave - essentially self-esteem amid seeing themselves as outcasts.
Are there any laws specifically aimed at gangs?
The government recently introduced gang injunctions in England and Wales - so-called "gangbos".
The injunctions can be used to ban people from certain places or from walking aggressive dogs.
The powers are similar to anti-social behaviour orders and were conceived by the former Labour government after appeals from councils that wanted to take the signs and symbols of gangs off streets. Ministers say the injunctions should not replace prosecutions of gang members involved in violent crime.
So what about prosecutions?
The Crown Prosecution Service has become increasingly adept at using the legal principle of "joint enterprise" in gang-related crimes - a key feature in the conviction of the two men tried for the murder of old Agnes Sina-Inakoju.
Joint enterprise (JE) can be used to prosecute somebody who was party to a crime even if they did not commit the deed or in cases where it is impossible to work out which of the suspects carried out that act.
The most famous case of JE was Derek Bentley - hanged in 1953 for the murder of a policeman. He was convicted for telling his accomplice, who pulled the trigger, to "let him have it".
Since 2008, more than 350 defendants have been prosecuted in just 116 murder cases. The Metropolitan Police has also run publicity campaigns warning teenagers that they will be prosecuted under JE for gang-related crimes.