A new record was set in New York City at 12.01am on Thursday: no murders had been reported in the city in 10 days. The milestone broke a nine-day record set in 2013, when none were reported during the same time of year, in the chill of January.

A New York police department spokeswoman confirmed to the Guardian that no murders were committed in the city on Wednesday night into Thursday.

But criminologists cautioned against reading too deeply into the statistic: crime, they say, swings with the seasons, and pinpointing what causes a dip is difficult.

“There’s a lot of social forces going on,” said Daniel Nagin, a professor of public policy and statistics at Carnegie Mellon University. “Crime rates, and homicides rates in particular, have declined very substantially since the early 1990s.”

The NYPD started counting days between homicides in 2003, a department spokesperson said, but the drop in crime started long before, as part of a nationwide trend of falling crime rates.

From 1994 to 2013, the most recent year for which the FBI has data, the US rate of crimes such as murder, rape and robbery fell by almost half, from about 713 violent crimes per 100,000 residents to 367 violent crimes per 100,000 resident.

Statistics released weekly by the NYPD show that crimes reported in the city have also fallen precipitously. Since 1993, murders reported are down 79.7%, robbery is down 82%, serious assaults are down 57.9%. Rape, a statistic complicated by victims’ reticence to report such attacks, is also down 52.7%.

26 November 2012 became famous when, among New York’s more than 8 million residents, no violent crimes at all were reported.

“There’s been much discussion about the reason for the declines among criminologists, economists, about why that happens,” Nagin said. “On that issue, I’ve never found any of the explanations particularly convincing.”

Nagin and other sociologists say that many theories are flawed. Hypotheses on the dip in violent crime have ranged from the availability of legal abortion to the concentration of lead particulate in a given neighborhood.

“One reason they fail to explain is: why is it homicide rates have been declining in many places across the world?” Nagin said about the theories generally. “In most of western Europe, homicide rates have declined by 50%.”

While the phenomenon befuddles criminologists, politicians are eager to take credit. For many public servants and police, the dropping crime rate is a bright spot amid renewed scrutiny of police tactics, especially violence between officers and the public.

“Keeping New Yorkers safe from harm is our top priority,” said New York’s mayor, Bill de Blasio, in January, touting the cities record low crime rate. “Our officers’ commitment to safe neighborhoods is clearly reflected in last year’s record low crime rates … we’ve worked together to achieve these extraordinary gains.”

Eli Silverman, a professor emeritus at New York’s John Jay College of Criminal Justice, pointed out that homicide generally dips in the winter.

“These things are cyclical,” Silverman said. “This whole issue of the crime drop is a controversial issue … Who should be accorded what credit, and what factors occurred is very difficult to sort out.”

Silverman said that police work is important, but that it’s part of a larger picture where the community and police work together.

“It’s an interactive process. One feeds on the other, and one supports the other, so as far as slicing it down between these different elements is very difficult.”