“Why is there a rise? No one can possibly know that,” said Robert Smith, director of the Fair Punishment Project at Harvard Law School. “But crime statistics are most accurate when they are block by block, because even a neighborhood is too large.”

Still, attempts at explanations are numerous: battles over drug dealing turf; the dissolution of once powerful street gangs, resulting in violent crews that disband as quickly as they form; petty disputes that turn deadly because of the ready availability of guns; and a deepening economic and social isolation of the nation’s poorest.

Crime experts say that studying a single year of crime data — instead of a decade or 20-year periods — reveals little about crime, especially if a particular year runs counter to the current long-term trend of falling crime.

“Crime rates are well known to be volatile, and year-to-year variation does not really tell you much,” Dr. Berk said. “The reasons are that crime rates are driven by many factors at the neighborhood and even interpersonal level that we do not measure and about which we have little understanding.”

Inimai M. Chettiar, director of the Justice Program at the Brennan Center for Justice, a nonpartisan public policy group, said that the nation might have entered a period in which some types of crime, particularly homicides, would regularly seesaw from year-to-year in certain cities, but neither rise nor drop significantly.

“There have been predictions that as crime decreases, we will see more up and down movements, but that crime itself will stay relatively steady,” she said.