Key factor in murder trends: youth, gang violence.

Some cities are seeing a drop in the number of officers on the beat and a shift in police resources to fight terror.

Murders are down to a 40-year low in New York and Chicago. Yet homicide rates are on the rise in Baltimore and Detroit – and dramatically so in New Orleans.

In that variance is a positive story about cities' successful attack on crime and gun violence, but also an alarm about rising gang-related and youth violence, particularly within the African-American community.

Across the US, the incidence of intentional killing is still relatively low compared with the early 1990s, when crack and gang violence had a deadly grip on America's large cities. Indeed, the homicide rate is almost half what it was then. But it is ticking up – in some cities quickly because of surges in the number of young people involved in gangs.

An analysis of federal crime data by Northeastern University's James Alan Fox found a 52 percent jump in the number of murders committed by male African-American teens between 2002 and 2006, and smaller increases in those committed by black men and women. In contrast, the number of killings committed by whites of any age during the same period showed no increase.

"Some cities are experiencing gang problems that are spiraling out of control, and others are not," says Professor Fox. "That's essentially the issue."

Criminologists cite a variety of factors for the increases, from a drop in the number of officers on the beat to a shift in resources to fight terror to cuts in federal spending on youth programs and gang prevention. Alfred Blumstein, a criminologist at Carnegie Mellon University in Pittsburgh, also cites a lack of economic opportunity for young people.

"The phenomena are very different and local across the country: There's now no major national drug epidemic like there was in the early 1990s," says Professor Blumstein. "All politics is local. Also, these days, all crime is local, too."

Still, it's clear that the availability of resources for police and prevention programs – as well as economic development – plays a key role in determining which cities have higher homicide rates. Take New York, at its 40-year low, and compare it to Baltimore, which is at an eight-year high.

"New York has more money for youth programs, prevention programs, and things like summer jobs, so they're able to control their gang problem," says Fox. "Whereas other cities like Baltimore don't have the resources to offer the same alternatives."

Since 2000, cities with populations over 250,000 have seen a 10 percent drop in the number of police because of budget cuts, according to Fox. Some of those resources have shifted to homeland-security efforts. In many medium-sized cities, such as Baltimore, that shift has been accompanied by a drop in resources for antidrug and antigang efforts.

Although New York is operating with fewer officers on the beat than in 2000, and is spending significantly more on antiterrorism efforts, it still has enough resources to regularly analyze crime data and shift officers to high-crime areas. It has also aggressively worked to get illegal guns off the street. Baltimore, on the other hand, has only recently revived a police unit that targets illegal guns.

"The criminal justice system is also insanely lenient here [in Baltimore], particularly with offenses involving firearms," says Daniel Webster, codirector of the Center for Gun Policy and Research at the Johns Hopkins Bloomberg School of Public Health. "I don't think judges here appreciate that the person in front of them today for illegally possessing a firearm, next week could be in front of them as a defendant in a murder case. Everyone points the finger at everyone else. It's a sad state of affairs, but it is fixable."

New Orleans has by far the highest homicide rate in the country. Even before hurricane Katrina devastated the city's criminal justice system, murder was a serious problem, according to Tulane University's Mark VanLandingham. In 2004, most cities its size had a homicide rate of about 13 murders per 100,000. In New Orleans, the rate was 57 per 100,000 people, four times higher. Since Katrina, the murder rate has soared even more, in part because the overall population has shrunk while the number of murders has remained high. A conservative estimate of the population puts the 2007 murder rate at 76.4 per 100,000, according to Professor VanLandingham.

"Even at 76 murders per 100,000, it's still nine times higher than most cities our size and 34 percent higher than it was before Katrina," says VanLandingham.

And that's with the National Guard and state police patrolling the streets along with local police. Experts say many murders have resulted from an influx of drugs, guns, and dealers.

"The murder risks have outstripped the population growth, and that's what's frightening," says Prof. Peter Scharf of Texas State University at San Marcos. "[The city's] crime strategies have broken down. People aren't going to put up with it."

Across the country, the increase in guns and gangs has also taken a toll on police. In 2007, 186 officers were killed in the line of duty, compared with 145 in 2006, according to the National Law Enforcement Officers Memorial Fund. With the exception of 2001 with its terrorist attacks, that makes 2007 the deadliest year for police officers since 1989.