By Steve Swann BBC News, Chicago

On a bitterly cold night on the city's West Side police officers from the gang intelligence unit patrol in a convoy of unmarked cars.

The number of murders in Chicago has reached a 40-year low

A group of young men on a street corner scatters but the officers catch up with them down an alley piled high with banks of snow.

One of them admits he is a member of a local gang, the Conservative Vice Lords.

No drugs are found, so he and his friends are eventually allowed to go.

With an estimated 68,000 gang members in Chicago, the city's 13,000 police officers have a difficult and sometimes deadly task.

So far this year there have been 430 murders.

That is a 40-year low, but still more than double the number of homicides in London, which has more than twice the population.

Gang culture

One of the casualties in this war of attrition between gangs was 20-year-old Tramell Toles.

He was a Conservative Vice Lord in a district run by the Four Corner Hustlers.

They caught up with him and shot him dead.

His mother Linda says: "A week or so earlier he had done a drive-by shooting so I guess it was retaliation. He was shot dead at 11am in broad daylight. No-one saw a thing."

Linda Toles thinks her son was killed in retaliation for a shooting

Gang culture pervades much of the West and South Side of the city.

The African-American gangs tend to fight over drugs and money while the newer Hispanic gangs appear to be more hierarchically structured, battling for territory and status.

The police claim credit for taking many of the gang leaders off the streets by the better targeting of gang hotspots.

In weekly briefings at their Deployment Operations Center they share intelligence and in the words of Michael Cronin, deputy chief of Narcotics and Gangs "put manpower where the violence is".

But despite some of the strictest gun control laws in the US, the urban skirmishes rumble on in Chicago.

'Violence interrupters'

The intensity of the killings has led to the creation of CeaseFire, an organisation aimed at mediating conflicts.

We didn't choose this life... It chose us Gang member

It is the brainchild of Dr Gary Slutkin, an epidemiologist who ran campaigns to cut Aids deaths in Africa. He says he is applying a behavioural model learnt there.

"Violence is learnt behaviour. There's an infectious nature to violence, but it's preventable."

Former gang members are sent out to hotspots to mediate conflict.

CeaseFire calculates it has stopped 240 potential killings in 2005.

One of its mediators or "violence interrupters" is Juan Johnson, who had a first-degree murder conviction overturned after serving 11 years in a federal penitentiary.

He speaks a language that some of the angry young men of Chicago understand.

In an office on the West Side he meets a group of Hispanic gang members.

One of them, a 35-year-old father of three, says: "We didn't choose this life. It chose us."

Another admits: "I'd have killed someone but for CeaseFire."