Stacy Spell is trying to save lives. The 20-odd youngsters ranged in a semicircle in front of him are healthy enough. But most are on a path towards early death.

“If any one of you drops a body, your whole group will be targeted,” Spell booms, pacing the room like a public defender in a courtroom. “Even if you weren’t there, we will come after you.”

The youths don’t move. All have criminal records. Most sit slumped in their chairs. One man with big, brown eyes wears his jacket with the collar so high that the zipper reaches his nose. His hands are buried deep in his pockets.

“Take this message back to the barber shop, take it back to the club, take it back to your crew,” says Spell, a former homicide detective in New Haven, Connecticut, who is now part of a programme with a very different take on crime and punishment.

It’s known as group violence intervention (GVI), an attempt to forestall violence in US cities developed in Boston in the 90s by David Kennedy, a criminology professor. When the city’s youth homicide rate fell 63%, it became known as the “Boston miracle”.

Today, Kennedy’s team at John Jay College of Criminal Justice in New York supports more than 30 American cities. New Haven first tested GVI in 2012. The year before, there were 349 shots fired in the town of 130,000, and 27 people died. In 2017 it was 110 shots, killing six people.

A young man is arrested by New Haven police officers. Photograph: Rob Schoenbaum

The success of GVI has been such that European countries are starting to take note. This session was attended by police officers from Malmo, Sweden, who are interested in following suit. Glasgow has established its own variant of GVI, offering young men a way out of crime through education, training and mentoring – delivered by people with experience of street violence.

London’s mayor, Sadiq Khan, recently announced an anti-violence unit modelled on Glasgow’s in a bid to curb knife crime.

“We are eager to try,” says Glen Sjögren, Malmo’s police commissioner. “If we can make the shootings go down we can regain trust in the community. Our biggest problem today is that no one dares to testify.” He hopes Malmo’s success will inspire other Swedish cities to adopt the strategy.

The GVI strategy is based around “call-ins” like the one Spell was addressing: a gathering of law enforcement officials, locals, former gang members, and young men attending as part of their probation or parole.

Spell gives the men a stark choice. Stop the violence and a whole range of assistance will come their way: nappies for the kids, the right paperwork for a job, even help with relocation to get out of the damaging environment in which they live. Carry on, and prison awaits – or worse.

Relatives and representatives of local services also attend. At the New Haven meeting, one social services coordinator, Letitia Charles, pointed at the audience as she asked the young men: “Have you ever heard it takes a village (to raise a child)? Look around you. You have a village.”

The dialogue approach is based on carrots and sticks. The police also use peer pressure dynamics in criminal gangs to reach all members, not just the individuals present at the call-in.

This meeting at the headquarters of New Haven police department is open to the public. Members of local, state and federal law enforcement attend, reporting on local issues and investigations, and displaying pictures of individuals they are seeking Photograph: Rob Schoenbaum

Anthony Campbell, the chief of police, tells them he doesn’t want to attend another funeral. Lieutenant Herb Johnson presents some of the intelligence police have on a couple of local gangs. On a board, he shows the faces of some young criminals. “Sentenced to 20 years in prison”, reads a caption under one of them.

Among the victims in 2011 was Sean Reeves’s son. He was hit in the neck by a stray bullet during a sudden street fight; he never turned 16. Reeves attends the call-in to talk about the pain he awakens to each morning. Reeves himself once lived the criminal life; at the age of 21 he lost his best friend to the streets.

“I had women, jewellery and cars,” he says. “But I also missed the birth of my first daughter, my son’s first baseball game and my parents’ death.”

After his speech, he says it’s impossible to reach them all: “To me, it’s about reaching one.”

They obviously reach more than that. So far this year, shots fired in New Haven are down by almost 80% from 2011. Chicago experienced a 32% reduction in victims among the gangs represented at call-ins, New Orleans a 32% decrease in gang homicides and Stockton a 55% reduction in homicides.

Last year, Kennedy’s team started to spread their research to Europe and Central America. Juárez, one of the world’s deadliest cities due to drug cartel wars, is among three Mexican cities implementing GVI. Police in El Salvador, seeking a way to handle Mara Salvatrucha or MS-13, a brutal gang wreaking havoc there, are also interested.

At the end of the meeting, Spell gives each attendees his business card and shakes them by the hand. He tells them to call anytime.

Detective Stacy Spell officiates at a call-in of the group violence intervention programme in New Haven. Photograph: Rob Schoenbaum

Almost a decade ago, Ducamel “Duke” Denis, 32, received a business card from Risco Mention-Lewis – now the city’s deputy police commissioner, everybody calls her Ms Lewis – at a call-in on Long Island, New York. They still talk at least once a week.

Duke joins a regular support group at a resource centre run by Lewis. They get their coffee from 7-Eleven and sit on folding chairs. Lewis and her co-workers know everybody by name.

Duke was arrested for the first time at the age of 16, in 2002. When he walked into the call-in in 2009, then called Operation Ceasefire, he was on probation for stolen property and gang assault and had done some prison time.

“I knew I never wanted to go back. When they showed me everything they knew, I thought I was either going to be the best criminal I can ever be or quit. And I remember they said: ‘We don’t want to lock you up.’”

Arrested in the past, Duke knew the police officers who were there.

“I hated them back then. I don’t hate them any more.”

Today he is working, studying and has his own photography business. He got into a couple of bad situations after the call-in, but calls it “the eye-opener”.

Ducamel ‘Duke’ Denis was first arrested at 16. After a string of gang-related arrests he was asked to attend a call-in, which proved a turning point for him Photograph: Rob Schoenbaum

He grew up with six sisters and his parents divorced when he was a teenager. He quickly joined one of the local gangs in Long Island. Reminders are everywhere; he still has a bullet in his lower back from when a rival gang “decided to go hunting”. Even now, he still bumps into former rivals sometimes.

“You build bad blood with people. They view you as the person you were. I still remember when they did something to my family and brought tears to my eyes. Some people probably don’t think I deserve to live.”

He drums his fingers at the table when he talks about his past, like he is impatient to change the subject. He admits he used to be tempted to go back to his former life for money, adrenaline and respect; “to feed my ego”. These days, people rarely freeze when he walks into a supermarket.

“But my kids care that I’m there,” he says with a smile that appears every time he talks about his son and two daughters.

“I feel like a get a new chance every single day.”

Additional reporting by Libby Brooks in Edinburgh

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