In Baltimore, according to Police Commissioner Edward Norris, the city's violent crime has jumped because beat cops are "distracted" by counterterrorism efforts added to their job descriptions.
In Philadelphia, officials ascribe their jump in crime to reassigning officers to security detail, checking suspicious packages for anthrax or bombs.
But Boston and New York officials say many violent crimes are down due to new community watchfulness.
Ordinary Americans have become more vigilant. But police are suddenly stretched thinner as terror probes compete with traffic patrol.
Criminals are being faced with a new environment, as well. for example, tighter borders have lessened illegal drug supplies, 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 ending.
"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."