As homicide rates inch up in cities around the nation, top cops are scrambling to find ways to combat what could be the start of a new crime wave. After nearly a decade of decline, homicide rates in Phoenix and Boston rocketed up 60 percent last year. Houston, St. Louis, Atlanta and San Antonio also saw homicides jump between 10 and 20 percent. The solution, say chiefs and criminologists, may not be easy or universal. Although in most places homicides are down from the drug-fueled crime wave of the early '90s, and in some cities like New York the decline has continued, experts aren't sure what's behind the recent uptick in violent death.

"That's what's driving us nuts," says police spokesman Patrick Camden in Chicago, which has the highest per capita murder rate in the nation. "I wish we could pinpoint where it's coming from, but we can't."

Some experts blame a sagging economy, a demographic bulge of young people and the large number of violent criminals being released from prison. Others point to a spike in homicides after 9-11, suggesting that criminals took advantage of local departments burdened by national-security concerns.

A few cities have initiated measures they hope will stem the tide. Boston Police Commissioner Paul Evans wants to stamp out what he calls "up close and personal" homicides, where perpetrators and victims are known to each other and the shootings are often retaliatory. His department is ramping up investigations into fatal and nonfatal shootings alike. "The fear is, if we don't apprehend the suspect," says Evans, "people take care of it themselves."

Seeing the same up-close and personal trend, St. Louis has started sending police officers to hospitals after shootings to identify friends of victims and to keep close tabs on them. "We want to stay on top of people who might get caught up in retaliatory violence," says St. Louis chief Joe Mokwa. "If we can't change their behavior, at least we can make sure they aren't carrying a gun."

In Phoenix, homicides dropped two years ago when the federal government tightened borders with Mexico, but, police say, spiked up again last year as narcotics traffickers learned to circumvent the Feds. "The common denominator for many of these killings is drugs," says department spokesman Det. Tony Morales. Cops there, he says, have plans to beef up narcotics enforcement.

In Chicago they're hoping their year-old accountability office, which makes commanders answerable for crimes that occur on their watch, will give police the edge they need. But police spokesman Camden admits t's not a sure fix. "The truth is, homicides are difficult to prevent. I we could find the cure, we'd patent it."