Among the reasons given: gangs, drugs, the easy availability of illegal guns, a disturbing tendency among young people to pull guns when they do not get the respect they demand, and, in Houston at least, an influx of Hurricane Katrina evacuees.
In New York, where the city reported 579 homicides through Dec. 24 — a nearly 10 percent increase from the year before — the spike is mostly the result of an unusually large number of "reclassified homicides," or those involving victims who were shot or stabbed years ago but did not die until this year. Thirty-five such deaths have been added to this year's toll, compared with an annual average of about a dozen.
At the same time, Police Department spokesman Paul Browne noted that this year's total is only slightly higher than last year's 539 homicides — the city's lowest death toll in more than 40 years.
Browne blamed the rise in part on the availability of guns, particularly weapons from out of state. The city this year sued dozens of out-of-state gun shops that it says are responsible for many of the illegal weapons on the streets of New York.
In Chicago, homicides through the first 11 months of the year were up 3.3 percent compared with the same period in 2005, reversing a four-year decline. A police spokeswoman said gang violence has been a contributing factor.
In New Haven, Conn., there were 23 homicides as of Tuesday, compared with 15 in 2004 and in 2005. Police Chief Francisco Ortiz said that about half of this year's killings involve young people settling disputes with guns instead of fists.
"They're all struggling with this thing about respect and pride," Ortiz said. "It's about respect. It's about revenge. It's about having a reputation. It's about turf and it's about girls."
Houston police attribute the 15 percent increase in the homicide count to the influx of Katrina evacuees from the Gulf Coast.
"So we expect that to settle," Lt. Murray Smith said. "We're hoping it will go down."
New Orleans, with its post-Katrina exodus, is the only major U.S. city that saw a sharp decline in the number of homicides. There were 154 in New Orleans this year as of Monday, said police spokesman Sgt. Jeffrey Johnson, down from 210 in 2005. But the city was largely empty during the fall and winter of 2005-06, and even now has only about half of its pre-Katrina population of 455,000.
Some cities, like Cincinnati — which has had 83 homicides so far, up from 79 in 2005 — posted their highest numbers ever. Others saw their highest death tolls in years.
Oakland, Calif., had 148 homicides as of Wednesday, up 57 percent from last year and the highest in more than a decade. Philadelphia's 2006 homicide total was 403 as of Wednesday, the first time the number has topped 400 in nearly a decade. There were 380 killings in all of 2005.
Philadelphia officials have struggled all year to reduce the violence. In July, Mayor John F. Street gave a televised address in which pleaded with young people: "Lay down your weapons. Do it now. Choose education over violence."
A few cities reported slight decreases in murders. Los Angeles' total was down about 4 percent to 464 homicides through Dec. 23. San Francisco's fell about 15 percent. San Francisco Police Sgt. Steve Mannina said the drop is partly due to increased patrols in violence-prone areas and more overtime approved by the police chief.
The FBI does not release its national crime statistics until several months after the end of the year. The bureau's statistics for the first six months of 2006 showed an increase of 1.4 percent in the number of murders in the first half of 2006 compared with the first six months of 2005.
Andrew Karmen, a criminologist at John Jay College in New York, said that while there are various theories for the drop in murders in New York and other cities in the 1990s, no one knows for sure why it happened. And if they are going up again, no one knows the reason for that, either, he said.
He noted that police departments tend to take credit when the murder rate goes down. "When crime goes up, it will be interesting to see whether they will accept responsibility," Karmen said.