In Allegheny County, an area of 730 square miles and 1.2 million people, there were 120 murders last year.

In the Hill District's Addison Terrace apartments, an area of only a few blocks, there were five murders, nearly one for every 100 households.

All five victims were black males. All were shot to death: Shawn Robinson, 32; Paris Tatum, 18; Maurice Wright, 18; Troy White, 32; and Kenneth Woods, 30.

It was the second bloodiest year on record for the county as a whole, but the violence barely touched many municipalities and Pittsburgh neighborhoods. Others experienced a relentless barrage of gunfire.

Today, post-gazette.com is posting a Google map of Allegheny County that displays the location of every 2008 homicide, with a red pin marking each spot. A mouse click on the pin brings up a bubble with the victim's name, age, race and cause of death, and links to Post-Gazette stories.

The pins are concentrated in areas that long have been plagued by drugs and gun violence. But the map also shows unique patterns.

The heaviest concentrations of homicides in the county are found in the city neighborhoods of Homewood and the Hill District, which are both predominantly black.

Homewood's nine killings are scattered throughout the neighborhood, while 10 of the Hill District's 12 murders are in or near the Bedford Dwellings and Addison Terrace apartment complexes.

Such concentrations of violent crime resemble the outbreak of an infectious disease. Local officials must develop a comprehensive approach, beyond law enforcement, to address the underlying symptoms, argues Dr. Stephen Thomas, director of the Center for Minority Health in the University of Pittsburgh's Graduate School of Public Health.

"We need to recognize that violence is a public health problem," he said. "We need to do more than simply count the bodies."

The map also shows the risk of living in certain parts of the county, even if one avoids the drug trade or other illegal activity.

Dr. Anthony Fabio, director of the Center for Injury Research and Control in the Department of Neurosurgery at Pitt, has spent the last five years studying the connection between segregation and violent crime, with the help of a $500,000 grant from the Centers for Disease Control and Prevention.

Both black people and white people in Pennsylvania face a greater danger of becoming victims of violence if they live in highly segregated communities, he and his research partners show in a paper that will be published in April by the American Journal of Public Health.

The danger is heightened even after setting aside other factors such as income and age. For black people in highly segregated communities, the risk is even higher, his findings show.

For decades, experts have speculated about the reasons behind violence -- broken families, poor schools, a lack of job opportunities. But there is little hard data on how segregated communities become a fertile ground for violent crime.

"I'm not sure we really understand it," Dr. Fabio said. "There's some social process going on because these neighborhoods are isolated."

Pittsburgh remains a segregated area, despite decades of improvements. In 2001, a report from the Brookings Institution said more than 68 percent of the metropolitan area's black residents would have to move to achieve an even representation of race in every census tract. The report called such a percentage "hypersegregation."

The middle Hill District, for instance, had a population of 2,143 when the 2000 census was taken, and nearly 97 percent of the population was black.

The segregation level is even greater in Addison Terrace, the oldest community overseen by the Housing Authority of the City of Pittsburgh.

A collection of aging apartment buildings clustered around Elmore Square in the Hill District, the complex has 1,009 residents, according to housing authority figures. Of those residents, just two are white.

Few people have deeper community roots than 26-year-old Rob Jamison. He has spent his whole life in Addison Terrace. His two brothers and two sisters all live there. His father is from Elmore Square, and his mother is from nearby Bentley Drive.

Mr. Jamison also works in the community. He's a local program manager for YouthPlaces, an organization that mentors at-risk youth throughout the city. Every afternoon, about 40 kids come to the organization's cramped room in the Addison Terrace complex to do homework or use four computers.

Mr. Jamison didn't have a similar site when he was growing up. But he did have an advantage over many friends -- he was raised in a two-parent home.

"That molded me and showed me how to be a man," he said.

His love of playing sports, especially basketball, also kept him out of trouble. But he's not immune to the violence.

As a teenager a decade ago, he was a camp counselor for Paris Tatum, then an energetic 8-year-old who also loved sports and "just enjoyed life," Mr. Jamison said.

Mr. Tatum was shot to death in the 300 block of Elmore Square on Jan. 11, 2008. Three people have been ordered to stand trial in his death.

"We all grew up together, and we all know each other. If something like that happens in the community, it affects all of us," Mr. Jamison said.

His sister, 22-year-old Ieasha Jamison, said Mr. Tatum used to come to the YouthPlaces site to use a PlayStation. She saw him walking through Elmore Square just a few hours before his death.

Another of her brothers was friends with Kenneth Woods, who was shot and killed on Bentley Drive on June 16.

"We've lost a lot of people," said Ms. Jamison, who also works for YouthPlaces and has an 8-year-old son and an infant daughter. "I want to see a different environment for my kids."

She hopes to move someday to Highland Park, one of Pittsburgh's most diverse neighborhoods.

Rob Jamison, with four children of his own, has no plans to leave Addison Terrace.

He sees a lack of jobs as one of the community's core problems. Many residents, both older and younger, are pulled into illegal activity when they can't find any other way to pay their bills.

Last summer, when his organization offered to pay local youths $250 a week to help renovate several basketball courts in the neighborhood, men in their 30s started asking to participate. Mr. Jamison told them the program was for children, not adults.

"They said, 'I just need some work.' "

Pittsburgh police Chief Nate Harper also sees employment as a critical factor in combating violent crime.

Chief Harper, 55, grew up in the Hill District, and he sometimes attends services at Macedonia Baptist Church on Bedford Avenue. As a teenager, he stocked shelves at a grocery store on the corner of Milwaukee and Clarissa streets.

He remembers walking through the neighborhood during the riots that followed the assassination of the Rev. Martin Luther King Jr. in 1968. He was tempted to join friends who were looting stores along Herron Avenue, but he feared his father's reaction if he brought home stolen goods.

Today, there are few businesses left in the Hill District. Many neighborhood residents have little hope of finding work elsewhere because they lack the educational background. Or they can't pass a drug test.

The solution to violent crime in such an environment won't come from an increased police presence, Chief Harper said. Community-based groups -- such as YouthPlaces and One Vision One Life, an anti-violence initiative based in the Hill District -- must be on the front lines, he said.

He credits those groups with helping the city avoid homicides in January.

Darryl Crosby, 58, died on Feb. 4 in the city's first official homicide for 2009. He was found stabbed to death in his apartment at 100 Hyman Place in the Hill District. The city's second victim, a man whose name had not been released yesterday, was shot and killed Friday afternoon near Kirkpatrick Street and Bentley Drive, behind the Addison Terrace management office.

There have been five homicides in other county communities this year.

Dr. Thomas of Pitt advocates a similar approach. His focus is on the specific health issues that tend to develop in high-violence neighborhoods, including diabetes, heart disease and depression.

Focusing on crime alone, he said, is akin to just giving pills to a 300-pound man who smokes and suffers from a weak heart. Public officials need to see neighborhoods as a whole, from abandoned buildings to failing schools, if they want to address homicide rates.

"Inequality is concentrated when people feel there is no way out and they're left to fend for themselves," said Dr. Thomas, who oversees Pitt's Healthy Black Family Project.

Both Dr. Thomas and Chief Harper see the election of the nation's first black president, Barack Obama, as a positive development. His campaign last year energized many people across Pittsburgh to become engaged in their communities.

"It was no easy feat for him," Chief Harper said. "He really showed that a person can have hopes and dreams, especially a young black male."

Both in the Hill District and Squirrel Hill, "they voted for the same thing -- change," Dr. Thomas said. "But the change must occur right where we live."