NEW YORK » Three police officers knocked on the apartment door of a 15-year-old boy. He had already been on both sides of a police blotter: shot and stabbed, but also arrested for robbery. He ran in an East Harlem gang and lived with his grandmother on the seventh floor of a public housing building, where the stairwells reeked of marijuana.

He was the type of teenager destined for trouble. And that was precisely why the officers were at his door on a recent winter night.

The New York City Police Department has embarked on a novel approach to deter juvenile robbers, essentially staging interventions and force-feeding outreach in an effort to stem a tide of robberies by dissuading those most likely to commit them.

Officers not only make repeated drop-ins at homes and schools, they will roll up on them in the streets, shouting out friendly hellos, in front of their friends. The force’s Intelligence Division also deciphers each teenager’s street name and gang affiliation. Detectives compile a binder on each teenager that includes photos from Facebook and arrest photos of the teenager’s associates, not unlike the flow charts generated by law enforcement to track organized crime.

The idea, in part, is to isolate these teenagers from the peers with whom they commit crimes — to make them radioactive.

"We are coming to find you and monitor every step you take," said Joanne Jaffe, the department’s Housing Bureau chief. "And we are going to learn about every bad friend you have. And you’re going to get alienated from those friends because we are going to be all over you."

The police also keep tabs in more covert ways.

Detectives spend hours, day and night, monitoring the Facebook pages and Twitter accounts of teenagers in the program, known as the Juvenile Robbery Intervention Program, or J-RIP, and of their criminal associates. To do so, detectives create a dummy Facebook page — perhaps employing a fake profile of an attractive teenage girl — and send out "friend requests" as bait to get beyond the social network’s privacy settings.

At the same time, officers seek to forge relationships with the teenager’s family, drawing them in with perquisites like a hand-delivered turkey on Thanksgiving Eve and toys and brand-name sneakers for younger siblings. Officers also provide tailored help, shuttling family members to doctor’s appointments, connecting them with alcohol and drug-abuse counseling and filling out applications for low-income housing, food stamps, child support and child care.

The approach in New York comes at a time when gang violence has been blamed for higher murder and crime rates in cities like Chicago and Detroit, prompting federal and local law enforcement authorities to contemplate new initiatives to try to quell the cycle of gang activity and violence.

New York’s program is no panacea to violent crime; only a few hundred teenage robbers will be in the program at any one time, all from East Harlem or the Brownsville section of Brooklyn.

Nonetheless, the city’s efforts have drawn notice; the Police Department has given presentations on its program at conferences from Monterey, Calif., to Washington, D.C.

"I’m not aware of any police department nationally coming up with the same strategy or replicating what the NYPD has done here," said David M. Kennedy, director of the Center for Crime Prevention and Control at John Jay College of Criminal Justice in Manhattan.

The program builds, in part, on Kennedy’s successful homicide-reduction strategy, called Operation Ceasefire, that began in Boston in the 1990s and was later implemented in scores of cities. But New York’s program has a different and more narrow focus: juvenile offenders who live within specific neighborhood borders yet commit robberies beyond those boundaries.

GETTING TO KNOW GANGS

It is the Police Department’s own brand of tough love.

"We tell these teens, ‘You have a choice,"’ Jaffe said. "You will not victimize anyone else. If you commit a new robbery or any other crime that is going to hurt people, we are going to do anything we can when you get arrested to put you in jail. Your friends will get out. You are not getting out."

Youths in the program are flagged in police and court databases, so if one is rearrested, the police and prosecutors will coordinate their response.

Officers who run the program said they recognized what they were up against. Of the 165 East Harlem juveniles currently in the program, 55 are members of a crew, a ganglike fraternity whose members are aligned with one housing project or another. In Brownsville, more than a dozen J-RIP kids have been shot, one fatally and another who survived his fourth shooting this past summer. Last month, a program teenager was jumped by rival gang members and stabbed three or four times. He remains paralyzed.

Many teenage girls in the program have babies.

"You’ll see 14-year-old mothers, 28-year-old grandmothers, 40-year-old great-grandmothers," said Lt. David Glassberg, who runs the program in Brooklyn. "It’s crazy."

Jaffe created the program in January 2007 after she noticed a spike in robberies in Brownsville, a neighborhood with 21 public housing developments within 2.2 square miles. She tried traditional policing strategies, like increased foot patrols, but the robberies persisted, she said.

She decided to identify every juvenile younger than 18 who lived in public housing and had been arrested for robbery, anywhere in the city. The result was a list of 106 teenagers linked to 132 robbery arrests in 2006. Only 24 percent of the robberies occurred on housing property — a distinction that was important to Jaffe, because stopping these teenagers in Brownsville would have a beneficial impact throughout New York City.

Part of that effort can be seen in the online work performed by Detective Patrick Kennedy and his partner, Officer Victor Ramos. They monitor the Facebook pages and Twitter accounts of dozens of J-RIP teenagers from computers inside a precinct station house in East Harlem. The sand-colored brick walls are papered with color photographs, printed from Facebook, of program teenagers posing, or "mobbing," with peers in crews with names like "Broad Day Shooters" and "True Money Gang."

Looking at one photo, Kennedy pointed out a J-RIP teenager who was flashing a crew hand signal; he was among the smallest of the group, all wearing designer Marmot and North Face jackets in lime green, purple, orange and electric blue.

"When they are all colored up like this in jackets and they go walking around other developments, that’s a problem," Kennedy said. "They call that mobbing."

"To be familiar with the J-RIP kid, you have to be familiar with the crews he or she runs with as well," Kennedy continued. "We know all of the kids. And as much as we know them, they know us."

For Facebook, Kennedy creates an avatar, typically the persona of a female teenager, and sends out "friend requests." Sometimes, accepted requests are followed by a come-on from the targeted teenager, such as an inquiry about where the "girl" lives or whether she wants to meet up.

Department rules bar the detective from engaging, but he and Ramos spend at least two hours daily monitoring the teenagers’ chatter — alert for talk of fights, party plans and criminal activities. If a program teenager is looking for trouble, Kennedy said he could often see it coming and hopefully intervene.

These concentrated efforts have helped produce results: Of the 106 Brownsville teenagers, only 14 were arrested for a new robbery in 2007. The success led the department to expand the program to East Harlem in 2009.

RESISTANCE TO CHANGE

Leonardo Agosto, 19, entered the program that year, overseen by two police officers whose guidance continues to this day. He grew up in East Harlem, raised by a single mother who struggled with mental illness. "My mom — she hasn’t supported me in certain ways," Leonardo said, using euphemisms to convey a childhood of mental and physical abuse.

He was 15 when he and his twin brother robbed two other teenagers near Central Park on an October 2008 night. Leonardo delivered the first blow, knocking one of the victims to the ground. "I hooked him — pow. His head went flying," he recalled.

The police later showed Leonardo a photograph of the victim’s swollen and bloodied face, the result of a fractured nose and broken jaw. The image, said Leonardo, has stuck with him.

Leonardo spent seven months in a juvenile detention facility. When he got out, the two officers, Gilberto Ortiz and Rafaela Rosario, began their intervention. They secretly paid for a cap and gown so Leonardo could participate in high school graduation ceremonies. They later put him on a bus to the State University of New York in Delhi. The town’s rural landscape, more than 150 miles from East Harlem, might as well have been the moon. He arrived for freshman-orientation weekend, greeted by a creek and an unsettling quiet. He recalled shutting his hotel room door; fear and pride welling up as he began to cry.

Leonardo has stayed out of trouble, but challenges remain. He dropped out of college, and returned to East Harlem with nowhere to go. He now lives in a Bronx homeless shelter, sharing a room with seven older men. But he has a paid internship with a Harlem-based community organization.

John Rivera, 19, who lives with his parents at the Van Dyke Houses in Brownsville, recalled how he was "chilling with the wrong crowd" at the time of his robbery arrest. His involvement in the program has made him unpopular with his former friends.

"Some of them were like, ‘Oh, you working for the cops,"’ John said. "But they just friends. Friends come and go."

Now he talks of the officers’ impact on his life. He was given a new pair of basketball sneakers — "My first LeBron’s," he announced proudly — and has been taking steps toward a GED, although his police mentors were upset that he failed to show up at the J-RIP trailer to prepare for the test.

"Stop messing around," Sgt. James Lawrence told John. "You keep telling me, ‘Yeah, yeah,’ every time I see you, and then I don’t see you," the sergeant said.

Not every effort pays off.

Last winter, while driving through Brooklyn, scanning the streets for teenagers in the program, Glassberg said he spotted a flash of orange — deer-hunter bright. It was not the $500 orange Marmot jacket that caught his eye; it was the reedy teenager wearing it. He said he immediately suspected that the teenager, a 17-year-old gang leader named Laquan, had stolen the jacket.

Glassberg dispatched a detective to investigate. For more than a year, the lieutenant had invested heavily in the teenager. He checked up on him at home, where he lived with a single working mother. He took him on trips to the Statue of Liberty and a Staten Island Yankees baseball game, got him a summer job and personally drove him to sessions with a therapist.

Laquan entered the program in 2010 after he struck a 14-year-old boy in the head and stole his cellphone. When Laquan was indeed arrested for stealing the jacket on that January 2012 day, Glassberg said the teenager’s reservoir of second chances had bottomed out.

"Everybody had a stake in this kid, and when he got in trouble again, I thought, ‘Well, that’s it,"’ the lieutenant said. "He used up all his chits."

The judge agreed to hold Laquan at Rikers Island, where he remains.

OVERCOMING SKEPTICISM

When Jaffe asked him to help start the program, Glassberg, 44, said he saw an opportunity to break the trajectory of those born into poverty and neglect, and winding up behind bars before their 18th birthday.

On a recent rainy evening, the lieutenant and his team of officers piled into an unmarked police van. At about 6 p.m., they parked in front of the Howard Houses, a boxy brick high-rise building in Brownsville, to visit a 17-year-old girl who has been in the program since last spring.

Police arrested her on a gun-possession charge when she was 15. Last March, she was arrested for stealing an iPhone and beating up the victim. She gave birth to a baby born about six months ago and stopped attending school.

The girl sat on a couch in the living room; her younger sister was slumped next to her, wearing an expression that conveyed petulance and boredom. Their younger brother, 10, sat at a computer, its glowing screen the only source of light. When the officers arrived, four other teenage friends scattered, disappearing down the hall to a back bedroom.

"Where’s your mother?" Glassberg asked. The girl said she was visiting a sick aunt. The lieutenant told her about a new day care that has agreed to take the baby while she is in school. "We are going to bring you over there this week or next week. It’s brand new, beautiful," he said. She stared at him blankly.

On his way out, Glassberg instructed the girl’s younger siblings to clean up their bedroom and reminded the boy, who sat nervously sucking on his T-shirt, to keep the peanut butter jar in the kitchen. "You know we have problems with some critters in this place," the lieutenant gently chided. Moments later, the boy appeared in the first-floor apartment window and waved goodbye.

This approach to law enforcement is rarely seen by residents in some of the city’s most crime-stricken neighborhoods, where tensions between police officers and residents, particularly over stop-and-frisk policing tactics, have put up walls not easily breached.

"In low-income areas, nobody really believes in the police," a Brownsville resident, Renee Smith, said. When officers first visited her apartment after her 16-year-old nephew was arrested for robbery, Smith was suspicious and bemused. She looked at the two baby-faced officers, standing earnestly at her apartment doorway, prattling on about the program.

"I’m thinking it was some sort of trick to get into your business and get you in trouble," Smith recalled in a recent interview.

Initially, doors slammed shut, often with an obscene gesture and a few choice words.

The two officers, Josh Carvajal and Richard Elliott, eventually won over Smith and her nephew, Ramell, whom she adopted at age 2 because her sister had a drug problem. "They are like a father figure, like big brothers. They make Ramell laugh. They make Ramell believe in himself," she said. Last month, Ramell earned his GED.

Smith, like others who have come into contact with the program, drew a distinction between these officers and the ones who patrol their neighborhoods.

When a sergeant, James Lawrence, showed up at a teenager’s apartment to drop off information about a free trip to Alaska, the teenager’s father brought up a recent stop-and-frisk police encounter.

"You know they ran up on me last night," said Anthony McCrae, 44. "I come out of the store and walked around the corner and I heard, ‘Hey you? You!’ And their lights were flashing and everything."

"Well, there’s been a problem over there with robberies," Lawrence offered, delicately.

"I’m not out here robbing nobody. Come on," McCrae said.

Carvajal changed the subject. "I forgot to tell you, starting next week, we are going to start to tutor your son. He needs some help in writing," the officer said.

McCrae, who got out of prison in 1995 after he served more than seven years for attempted murder, said he did not want his 15-year-old son to "go down that path."

"These brothers here — they real good," McCrae said to a reporter. "They try to help out. Get my son on the right track."