http://news.bbc.co.uk/1/hi/england/london/8345673.stm
Senior Scotland Yard officers have admitted they are deeply concerned about a rising number of gang rapes.
New figures revealed there were 93 gang sex attacks in London in 2008/2009, compared to 71 in 2003/2004, with others unreported, detectives believe.
The age of victims has fallen with 64% aged 19 or younger in the last year compared to 48% a decade ago, the Metropolitan Police Authority heard.
Police define a gang rape as a sex attack involving three or more people.
Commander Simon Foy, who leads the Met's homicide and serious crime command, said there was no doubt the "abhorrent" crime was under-reported.
"There is no doubt that the number of multiple offender gang rape offences is going up and we can say there is an increasing number of offences with four or more suspects.
"The greatest proportion of victims of this type of offence are under 19 and there is a very significant number under 15.
"Young people are particularly at risk of this type of offending."
The authority heard levels of gang rape were linked to overall youth violence.
Boroughs with the highest numbers of gang rapes include Lambeth, Croydon, Newham, Southwark, Westminster and Hackney.
The meeting heard workers trained to work with young people are based at centres for sex attack victims, known as Havens.
Officials based in Whitechapel and Paddington have also visited secondary schools and youth groups across London to dispel myths around sexual violence.
Cmdr Foy said: "What I do not understand is what motivates people to commit this particular type of offence in these particular circumstances.
"It is often clear why someone would carry a gun or knife. What is the propensity to commit these criminal offences and how does it manifest itself?"
There have been a series of high-profile convictions of teenagers for gang rapes in the capital over the past year.
Two men who assaulted a girl aged 16 and doused her in caustic soda, disfiguring her for life, had their sentences increased on appeal.
In another case a 14-year-old girl was repeatedly raped "as punishment" by nine members of a Hackney gang because she had "insulted" their leader.