Criminals are back at work in many American cities after a brief lull following the terrorist attacks.

"You've got to understand that some people do this for a living," said Officer Rick Eckhard of the St. Louis County, Mo., Police Department. "The president said go back and do what you do for a living, and since that's what they do, they're back doing it."

Cities like Kansas City, Mo., Fort Lauderdale, Fla., and Hartford, Conn., reported a slowdown in crime on Sept. 11 and in the days after. Many police departments did not have hard numbers but said the drop was modest in most places.

"That week it appeared that things did slow down somewhat," said Kansas City police spokesman Steve Young. "We're assuming that everybody was at home watching television, even criminals and potential victims."

But the trend didn't last long in many places.

"Crime goes on," said John Turner, a spokesman for the Sheriff's Office in Jacksonville, Fla., which also had a modest and short-lived drop in crime. "The criminals are going to do what they do."

On Tuesday, New York Mayor Rudolph Giuliani cited statistics showing that violent crime had plunged in the two weeks since the attacks. During the week of the attacks, violent crime was down 17.5 percent compared with the same period a year ago, and officials attributed the drop in part to stepped-up police efforts. New York had four homicides last week, versus 10 in the same week last year.

Boston police spokeswoman Mariellen Burns said 1,514 crimes were reported from the day of the attacks through Sept. 15, compared with 1,609 during the same time a year ago. Burns said she had no more recent figures.

In Philadelphia, Police Commissioner John F. Timoney said the crime rate has not changed since the attacks, but bomb threats are up. "Some whack jobs are out there calling these stupid things in," Timoney said. "We know it's a prank but still we've got to treat it seriously."

There is no easy way to determine why crime drops during a crisis, said John Galliher, a professor of sociology at the University of Missouri at Columbia. "I'd like to be able to tell you that criminals are patriotic and would stop committing crimes," he said, "but I can't tell you that."

John Kilburn, an assistant professor of criminology and sociology at Eastern Connecticut State University in Windham, Conn., noted that some crime rates dropped during World War II, with the murder level declining to about half of what it was during the Depression.

"Our aggressions are no longer on our neighbors, our partners or the guy next to us at the bar," Kilburn said.

In Hartford, which had been coping with one of its highest homicide rates in years, crime was down dramatically following the attacks, though police said they had no hard numbers.

"The whole mood on the street is very subdued, even among the drug dealers and criminal element," said police Capt. Mark Pawlina.