This study relies on numbers from cities, rather than yearly F.B.I. totals, which are typically released in the fall. The group collected similar numbers last year, and those numbers were largely borne out by the data from the Federal Bureau of Investigation.

Police chiefs say the trends in aggravated assaults are particularly alarming. They are often considered a better gauge of violence than homicides; the difference between the two is often poor marksmanship or good medical care.

“Had we not had some of the trauma rooms we have here in Rochester, our homicide numbers would be higher,” said Mayor Robert Duffy, who served as a police chief for seven years.

While murder rates hit 11-year highs in places like Boston, police officials note that they are not seeing the highs of the late 1980s and early 1990s, when crack cocaine fueled spikes, particularly in large cities. Some cities like Denver and Washington had declines in homicides.

Still, the overall trend is mirrored in other places not covered by the report. New York City, for example, which had enjoyed remarkable declines and seemed immune to the rising murder rate elsewhere in 2005, reported a 10 percent increase in homicides in 2006. In Chicago, which had been cited as another model of declining violence, homicides rose 4 percent from 2004 to 2006.

Police officials say the violence tends to happen among young men in their late teens and early to mid-20s. In some cases, it is random. But in many cases, it is among people who know one another, or between gangs, as a way to settle disputes. Arguments that 20 years ago would have led to fistfights, police chiefs say, now lead to guns.

“There’s really no rhyme or reason with these homicides,” said Edward Davis, the police commissioner in Boston. “An incident will occur involving disrespect, a fight over a girl. Then there’s a retaliation aspect where if someone shoots someone else; their friends will come back and shoot at the people that did it.”