WASHINGTON — In 1990, at the height of a decade-long crime wave that swept the nation, 2,245 people were murdered in New York City.

In 2014, police investigated just 328 homicides in the five boroughs – a precipitous drop of 85 percent that’s being duplicated in major cities across the country.

Preliminary figures suggest 2014 will continue a decade-long trend of falling crime rates, especially in major cities once plagued by violent crime.

Criminologists say the decrease is linked to several factors, some of which are the product of smart policing, others completely out of authorities’ control.

But they also say the lack of a consensus on what’s gone right has them convinced that crime rates could spike once again.

“I don’t think anyone has a perfect handle on why violence has declined,” said Harold Pollack, co-director of the University of Chicago Crime Lab. “So everyone is a bit nervous that things could turn around.”

But the numbers are encouraging:

Chicago recorded an all-time high of 504 killings in 2012, but just two years later homicides were down to 392, and the overall crime rate has declined to its lowest rate since 1972.

Charlotte, North Carolina, recorded 42 killings last year, the lowest number since Mecklenburg County began keeping records in 1977.

Philadelphia’s murder rate has declined from 322 in 2012 to 245 this year.

Just 19 slayings were recorded in San Jose, the nation’s 11th-largest city, down from 24 the year before.

Even crime-plagued Detroit, which has one of the highest murder rates in the country, is improving: The 304 homicides recorded this year are down from 333 in 2013, the lowest rate since 2010 and the second-lowest number since 1967.

In the first half of the year, Phoenix police investigated just 43 homicides, down from 52 in the first half of 2013. Kansas City, Missouri, was on pace to reach its lowest rate since 1967.

Mid-year statistics in Dallas showed the city on pace to record just half the murders of its peak in 2004.

Camden, New Jersey, has seen the number drop by more than 50 percent since 2012. Murders in Columbus, Ohio, hit a six-year low.