What's at the root of Boston's rise in murders?

When Bus No. 22 takes Marlon Thompson home from school each day, it also takes him on a tour of Boston's swelling crime wave.

It rumbles past the grassy fields of Harambee Park, where he says a classmate was stabbed last year.

Then it rolls past Franklin Park, where a young woman's body was found badly burned last month. Later, as the bus approaches his home in Roxbury, Marlon points to a corner where a friend, while on a double date last year, got chased by a group flashing a gun.

"There are a lot more shootings that I've been hearing about," says the senior at Codman Academy Charter Public School in Dorchester, a Boston neighborhood. "It is crazy how it happens."

After years celebrating a dramatic dip in murders in what was hailed as the "Boston miracle," residents have been shaken by a resurgence of homicides and shootings concentrated in the city's poorer neighborhoods. Last week alone, police reported seven murders in seven days.

While Boston's murder rate remains lower than those of many major American cities, its 75 murders in 2005 marked its deadliest year in a decade. This year, 21 murders have already been recorded, two ahead of last year's pace, according to the Boston Police Department.

In response, Boston police have moved aggressively this week, beefing up patrols and installing surveillance cameras across crime hot spots. Last month, Mayor Thomas Menino joined 14 other mayors at a gun summit in New York aimed at stemming the tide of illegal guns.

The violence is felt most deeply by young people. In Boston and across the country, they are disproportionately the perpetrators of the crimes and its victims, according to the US Department of Justice.

The crime wave here is not solely attributed to young adults. But experts say that small bands of young men are playing a major role.

Criminologists and community leaders see several factors fueling the violence:

• Demographic shift. Typically, younger populations correspond with higher rates of crime. And Boston is experiencing somewhat of a boom of young people. As its segment of 15-to-24 year olds expanded, so has the area's violence.

• Funding cuts. Critics say that many of the federal, state, and local programs designed to allay violence among at-risk youth in Boston were cut back after the 2001 recession.

• Complacency. Boston's success in dramatically reducing violence a decade ago diminished the sense of urgency and vigilance among activists, allowing old problems to resurface.

• Violent street culture. Youth violence is often associated with turf battles between gangs or drug peddlers. But in Boston, experts suggest, many acts of aggression seem to be the result of petty disputes that take on deadly stakes when pride and reputation are on the line. "What you get is the rise of street culture in which respect and reputation ... and standing is everything," says David Kennedy, a criminologist at John Jay College of Criminal Justice in New York and a leader in Boston's anti-crime effort of the 1990s, dubbed "Operation Ceasefire."

Youths turn to guns today, he says, to deal with the normal social frictions that arise in their lives, a behavioral pattern learned during the drug epidemic of the late 80s. Of the 21 homicide victims this year, nearly half are under age 25. Last year 39 of the 75 residents murdered were also under 25.

It's a statistic not lost on Marlon and the students at Codman Academy. To memorialize last year's victims, Marlon and three classmates created a public art display as part of a school project. On a street fence, they hung 75 pairs of shoes on painted red boards with plaques bearing each victim's name. They had feared that the display might be vandalized, but instead, people have placed flowers, notes, and teddy bears. Relatives of victims have stopped by the school to express their gratitude.

Among their bigger worries, they say, is the fear of repercussion that keeps many would-be crime fighters from providing police with tips about suspects.

"If it were your family, you would want me to come forward," says Jarron Bennett, who helped create the wall tribute and attended middle school with one of the victims it honors. "But it's hard to do when there is no protection out here."

Boston's homicide rate is still far below the levels it reached 15 years ago. In 1990, the city recorded 152 murders. In 1996, "Operation Ceasefire" brought together police, clergy, courts, and citizens to tackle youth homicide. By 1999, the murders had dropped to 31.

But its resurgence has angered many community leaders and activists who lament a year of horrific headlines that include the death of Dominique Samuels, the 19-year-old former Milton High School cheerleader whose charred body was found April 30 in Franklin Park.

Reports of shootings on street corners seem to surface every weekend. This past weekend, the son of Isaura Mendes, a peace activist in Dorchester, was killed not far from where one of her other sons was murdered in 1995.

Community leaders are scrambling to respond.

Rev. Bruce Wall hands out fliers with pictures of coffins that urge teens who are traumatized or in "retaliation mode" to seek guidance. He received a call from parishioners last Sunday saying that a young man had been shot outside his church in Dorchester. "Teens are living in a community of 'yellow-tape,'" he says.

Across the street from his church, Meg Campbell, the head of Codman Academy, says that students at her school, while courageous and resilient, are dealing with the grief and stress of having to be "super alert" - a stress that has grown since the school opened in 2001.

She, like many others involved in the community, is worried about the summer, when crime tends to spike. This year, the school is offering a special program to put on a version of "Romeo & Juliet" with a city theater company, which will rehearse from 2 p.m. to 6 p.m. - hours especially prone to violence, she says.

For many area youths, guns have become a status symbol, says Jack Levin, a criminologist at Northeastern University in Boston. "All of a sudden you are a big shot," he says, "literally."

The number of nonfatal shootings to date has nearly doubled from the same period last year - to 137. And according to a police analysis performed for The Boston Globe, multiple shootings are up: 21 shootings with two or more victims were reported through April 30 this year, up from just four during the same period the year before.

For Marlon, who witnessed a shooting from the bottom of his stoop a few years ago, the sheer number of shootings is alarming - incidents he hears about from friends, neighbors, and chatter on the bus when he sits quiet and listens.

He says he sometimes feels paranoid walking alone, but at the same time, he has to be careful while walking with his three best friends from school, lest another group approach them and ask what "gang" they "represent." Mostly, he worries about mistaken identity - wearing the clothing of someone against whom there is a vendetta. But so far, he hasn't changed his daily routine. "This is reality, I have to live with it," Marlon says.

He says he can understand why some feel the need to join a gang - as many of his childhood friends have - but he never would. He has plans to study business at college next year. "I don't see the reason to join a gang," he says.

Still, sometimes he and his friends talk about whether carrying a gun is necessary for protection in this environment - especially when they get really angry about the violence around them.

Instead, he purchased a purple wristband with the word "confidence" on it. He says he bought it to give him the self-assurance to make life decisions - that his college major will take him on the right path and that everything will turn out OK.