Cities know that a majority of crimes are committed by a small slice of the population. In fact, somewhere between half and three quarters of all killings and shootings occur within about 3 to 5 percent of any city’s blocks. And a little more than half of all homicides are committed by 1 percent of the population, according to FBI data.

In recent decades, as cities have focused their efforts on reducing crime, many of them have launched anti-violence programs aimed at zeroing in on the small handful of residents most likely to commit those crimes. Those include efforts such as Project Longevity in New Haven, Conn., and New York City's Man Up, which was modeled after Chicago’s Cure Violence program, which uses violence interrupters to reduce the number of retaliatory violence in the city.

Sometimes, though, those kinds of initiatives aren't enough. And that's where something like Massachusetts' Roca program comes in.

Roca, which means "rock" in Spanish, is a unique type of crime-intervention program that focuses in on the riskiest of at-risk residents, the community's most troubled young men who won't take part in other programs and are the most resistant to change.

“Over the years we became very focused on that one specific group. All the effort we did here at Roca is designed to service them,” says Yotam Zeira, the group's director of external affairs. “These are the young people who are not willing, ready or able to attend another program. They actively refuse services.”

Now Roca is facing a new chapter with the launch of its first out-of-state location in one of the nation's highest-crime cities: Baltimore.

The Maryland city has been reeling from more than 1,000 homicides committed over the past three years. Its new Roca program -- a $17 million, four-year commitment -- is slated to begin this summer.

“I grew up in Baltimore, and like all of you, I am devastated by the violence in our city,” said Roca founder and CEO Molly Baldwin during a December press conference with Baltimore Mayor Catherine Pugh announcing the partnership. “As you are so painfully aware, we can neither arrest nor program our way out of this violence. We need a different approach.”

The target group for Roca is young men in their late teens and early 20s who have already come into contact with the criminal justice system. Roca's service workers work with law enforcement, as well as parole and probation officers, to identify the offenders who are the most at-risk, the ones least likely to accept help from other programs.

Roca builds ongoing relationships with participants in a therapeutic setting, teaching them coping strategies to solve problems and change patterns of behavior. Roca is also deeply driven by data: The group tracks each interaction and each outcome and uses the data to tweak its model.

But what most distinguishes the program from others is its persistence with clients. The outreach workers are often turned away by the clients on the first attempt to make contact -- or even the second or third. But the Roca workers continue to show up. They attempt to gain the trust of the young men the program targets for assistance.

"We coach our people" to be prepared when offenders refuse Roca's help, says Zeira. "We tell them they are going to have the door slammed in their face.”

Another distinction: Roca won't kick participants out if they slip up. With many other organizations, clients can only receive assistance if they're willing participants. They may even be kicked out of the program if they run afoul of the law again.

“We design services in a way to allow [clients] to come back and go and go through multiple relapses,” Zeira says. “We build relationships with them -- a real, deep relationship to help them change.”

Launched in 1988, Roca has made great strides in reducing criminal activity and breaking the cycle of poverty among young people in the Boston area and elsewhere in Massachusetts. It's now in five different sites in the state, serving 21 communities. Last year more than 850 young men participated in the program.

Of those who have graduated the Massachusetts program, 84 percent have no new arrests and two thirds retain employment six months after finishing the program, according to Roca’s own data.

Roca isn't infallible.

As the Baltimore Sun reported in December, "Two men were convicted last month in the death of a third man from a rival gang who was working on a Roca snow-shoveling crew. One of the attackers had also been on the crew, but had promised to work peacefully with the victim. Baldwin [the Roca CEO] said the organization tightened its security after the incident."

Still, its long track record has given officials in Baltimore hope that Roca may help stem the tide of crime there. The city already has engaged in many anti-violence efforts, including several ceasefire weekends, the most recent of which was a success, with no homicides committed during the period. Baltimore also deploys violence interrupters who attempt to resolve disputes before they turn fatal.

Pugh’s office has raised $6 million in private donations thus far to fund ROCA’s launch this summer. The city will spend $500,000 of its own money on administrative costs to run the program. The city may have to dip into its own funds to make up the difference. Pugh has called on the state to help fund the program, Zeira says. Gov. Larry Hogan, the Sun reported in December, had not committed state funds but hadn't ruled it out.

*CORRECTION: A previous version of this mistakenly stated that the Roca program in Baltimore would be entirely funded by private donations.