Mourners pay their respects Monday, Dec. 29, 2014, during a funeral for the victims of a quadruple homicide in Rockford, Ill. (AP Photo/Rockford Register Star, Max Gersh)

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, the 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, N.C., 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; final statistics for the Phoenix area haven’t been released yet. Kansas City, Mo., was on pace to reach its lowest rate since 1967, too.

Mid-year statistics in Dallas showed the city on pace to record just half the murders of its peak in 2004. Camden, N.J., has seen the number  drop by more than 50 percent since 2012. Murders in Columbus, Ohio, hit a six-year low.

The Federal Bureau of Investigation and the Bureau of Justice Statistics both collect crime data at the end of each year and issue reports throughout the year. Final statistics for 2014 won’t be available for several months.

But the trend lines are clear: The number of violent crimes has declined since 2006, according to the FBI’s Uniform Crime Reporting Program. The number of violent crimes committed per 100,000 people has been dropping even longer, from a high of 758 in 1991 to 367.9 in 2013. The rate hasn’t topped 500 per 100,000 people since 2001.

James Alan Fox, a crime statistics expert and professor of criminology, law and public policy at Northeastern University, pointed to four major factors contributing to the falling crime rate across the country:

— Long prison sentences, which have lengthened on average since sentencing reform initiatives in many states in the 1990s, have kept more criminals behind bars, albeit at a significant cost to state budgets.

— Improved community policing strategies are sending cops to places where crime is more likely to occur, as a prevention method. Technologies like video surveillance and acoustic sensors, which can hear gunshots before residents report a crime, are improving police response, too.

— A changing drug market has plunged the cost of heroin near historic lows, reducing crime associated with the drug trade. Pollack added that the end of the crack epidemic of the 1990s and 2000s has also contributed to a decline in drug-related violence.

— And an aging population is less likely to commit crimes. The fastest growing segment of the population is seniors, an age at which far fewer crimes are committed.

Academics advance other theories for the falling crime rate, ranging from the Supreme Court’s decision in Roe v. Wade legalizing abortion, the declining use of lead paint and improvements in medical technologies used in emergency rooms, which can save lives that would otherwise have been lost.

“Because the crime drop is being seen in so many places, one should be a bit skeptical of any particular police chief claiming that it is because of what his or her department is doing or any lawmaker claiming that some new legislation is responsible,” Fox said. “While local efforts may contribute, that the pattern is widespread tends to suggest global factors, not so much local initiatives.”

Not every major city is basking in the glow of lower crime rates. A rash of shootings between Dec. 23 and the end of the year brought the number of murders in Washington, D.C., to 105 in 2014, the second consecutive year of triple-digit murders, after the nation’s capital hit a half-century low in 2012.

The number of homicides in Los Angeles reached 254 last year, up four from 2013 and the first increase in 12 years. Those statistics may actually understate the real number: A Los Angeles Times investigation earlier this year found the Los Angeles Police Department misclassified about 1,200 violent crimes as more minor offenses in a recent one-year period.

Indianapolis, Austin, Pittsburgh, El Paso and Memphis all saw rates rise.

But even in El Paso, long ranked as America’s safest big city, there’s reason for optimism: While the number of murders rose from 11 in 2013 to 20 in 2014, crime rates in neighboring Ciudad Juarez, across the Mexican border, are falling. After recording an incredible 3,500 killings in 2010, the number of homicides fell to an estimated 424 in the last year, amid a dramatically increased presence by Mexican military forces aimed at stamping out the drug war.

“Declining crime implies a larger number of police officers per crime. So violence is easier to suppress. Crimes are easier to solve,” Pollack said. “If we are lucky, this is a self-reinforcing process.”