In Baltimore, Police Commissioner Edward Norris says violent crime has jumped because beat cops are "distracted" by counterterrorism efforts.
In Philadelphia, officials say reassigning officers to security detail - checking suspicious packages for anthrax or bombs - is to blame for a jump in crime.
But Boston and New York officials say many violent crimes are down thanks to community watchfulness and, perhaps, to heightened sensitivity in homes where domestic violence loomed.
Across the country, moves to counter terrorism are affecting crime rates in some subtle and surprising ways.
Ordinary Americans have become more vigilant. But police are suddenly stretched thinner as terror probes compete with traffic patrol.
Criminals, too, face a new environment. Tighter borders have crimped illegal drug supplies, for example, fueling turf-war violence in some areas.
Together, these changes add uncertainty at a time when some experts believe a decade-long decline in violent crime is drawing to a close. Murder rates, for example, are up this year in 7 of 10 large US cities surveyed by the Monitor.
"We are entering an era of general good will and good feeling, which has all the promise of contributing to a drop in crime rates," says Al Blumstein, a University of Pittsburgh criminologist. "At the same time, we are seeing police restricted into other missions than their usual street patrols.... How this plays out depends in large part on how Americans themselves pull apart or together."
Most crime experts caution about making too much of the upward or downward shifts of specific crime rates in isolated American cities since Sept. 11 - or jumping to early conclusions as to their causes.
Still, reports from city police officials offer clues about new factors at work as police add antiterror alerts to their traditional beats.
In Philadelphia, for instance, there was a 28 percent increase in murders during the four weeks from Sept. 14 to Oct. 11 in comparison to the previous four weeks. Robbery and auto theft were also up, by lesser amounts. Officials there say as many as 60 calls a day reporting suspicious packages, bomb threats, or anthrax concerns have taken hundreds of officers away from high-crime neighborhoods.
"While cops are being redeployed for undercover work to protect high-rises and bridges or government buildings, they are taxing the rest of the personnel they leave behind," says James Fox, a criminologist at Northeastern University in Boston. "Some of their highest-crime neighborhoods are being relatively unpatrolled."
But in New York, the site where the main terrorist attacks occurred and where police have been taxed to the limit, murder has dropped 11 percent over the same period. Other crimes, such as auto theft and assault, have dropped as well. Observers say this is the result of greater community vigilance but also can be attributed to intangibles such as fewer visitors and fewer parked cars.
"There has been a drop in victims to prey on," Mr. Fox says.
Likewise, in many smaller cities and rural areas - where local officials have not felt as pushed to instigate antiterror measures - the overall crime rates since Sept. 11 are not being noticeably affected. Nor are large cities in the West, such as San Francisco and Los Angeles, where concern about terrorist attacks has been high.
"We have not seen a significant increase or decrease in any crime category since Sept. 11," says Officer Jason Lee of the Los Angeles Police Department.
This despite high alert for more than 300 sites - including government buildings, bridges, churches, and mosques - and a 1,000-officer deficit in the police force. (Ranks now stand at 9,000 instead of 10,000.)
What Los Angeles has also seen, Mr. Lee says, is a spike in police-academy recruits, which he says exemplifies newfound concern among citizens for their communities. The recent entry rate has been 1,000 per month at the academy, up from 500 typically.
Beyond these city-to-city fluctuations since Sept. 11, the conventional wisdom is that crime will remain more or less on the same path it was on before the attacks.
"There is no question that key rates of violent crime across America are flattening out since our decade of dropping during the 1990s," says Blumstein. "The universal decline is behind us."
The reasons for that bigger shift are many. No. 1 is that crime rates can fall only so far. No. 2 is the economy, which has a domino effect starting with unemployment. Diminished opportunities add to the baseline impetus for crimes.
No. 3 is demographics. There is a small rise - 1 percent - in the number of baby-boom children reaching high-crime ages of 15 to 23. Meanwhile, the number of prisoners coming out of prison each year has grown from 450,000 to 500,000.
"People coming out of prison are particularly vulnerable to a tight job market," adds Blumstein.
One final note of concern, say experts, is that gains of the 1990s not be lost.
Demand for crack cocaine, for example, dropped during that decade in many American cities. That helped diminish crack markets and the young people recruited for them. Meanwhile, efforts to control the number of handguns held by such youths have in some ways declined. Progress on these key fronts requires effort to maintain.
"While others are trying to do a better job shutting off drugs at the border, that can create a more violent subculture in the 'hood between gangs fighting over smaller amounts of drugs," says LAPD's Lee. "At a crucial transition period like this, we are doing everything we can to focus on such trends to stop them before they get going."