The number of violent crimes in the United States dropped significantly last year, to what appeared to be the lowest rate in nearly 40 years, a development that was considered puzzling partly because it ran counter to the prevailing expectation that crime would increase during a recession.

In all regions, the country appears to be safer. The odds of being murdered or robbed are now less than half of what they were in the early 1990s, when violent crime peaked in the United States. Small towns, especially, are seeing far fewer murders: In cities with populations under 10,000, the number plunged by more than 25 percent last year.

The news was not as positive in New York City, however. After leading a long decline in crime rates, the city saw increases in all four types of violent lawbreaking — murder, rape, robbery and aggravated assault — including a nearly 14 percent rise in murders. But data from the past few months suggest the city’s upward trend may have slowed or stopped.

Criminology experts said they were surprised and impressed by the national numbers, issued on Monday by the Federal Bureau of Investigation and based on data from more than 13,000 law-enforcement agencies. They said the decline nationally in the number of violent crimes, by 5.5 percent, raised the question, at least in some places, of to what extent crime could continue to fall — or at least fall at the same pace as the past two years. Violent crimes fell nearly the same amount in 2009.