Image caption New York City has the highest number of murders in the US

FBI statistics show that US crime rates have fallen for the third consecutive year.

The data upends historical trends which suggests that crime rates increase during economic downturns.

Criminologists attribute the unusual decline to better policing and security technology, and programmes targeting repeat offenders and vulnerable youths.

Car thefts have fallen the most this year - 17.2% - while murders are down 7.2%.

Reported rapes also fell 3.1% and robberies are down 8.1%. Arson, assault and property crime have also declined.

All numbers are preliminary, and will be supplemented by local agency reports in the coming months.

New York City still had the highest number of murders, followed by Chicago, but those cities are both major population centres.

On a per capita basis, New Orleans, St Louis, Detroit and Baltimore fare poorly.

MURDER RATE DROP BY AREA Western states: -8.3%

Northeastern states: -8.0%

Southern states: -7.8%

Midwestern states: -3.8%

US total: -7.2%

The current recession has not prompted the increase in drug activity seen in previous downturns, such as the crack cocaine epidemic that began in the late 80s.

That may help explain the decrease in associated crimes including theft and assault.

Video surveillance and technological advances enabling more efficient monitoring and apprehension have also aided police and law enforcement authorities.

Cautious response

This year's drops were even larger than seen in the previous two years, but criminologists warn officials to remain vigilant.

"It's fabulous news, but I would draw an analogy to global warming: Even if you believe the long-term trend is increasing temperatures, it doesn't mean you can't have a cold year," Jonathan P Caulkins, a professor of public policy at Carnegie Mellon University's Heinz College, told the Associated Press agency.

But over the longer term, a slow recovery puts additional pressure on policing efforts and government programmes, meaning that downward trend may not be sustainable.

Northeastern University criminology professor James Alan Fox told the Associated Press agency that budget cuts created challenges in keeping crime rate low: "We have increasing numbers of at-risk youth in the population, and they need services. We need to reinvest in crime prevention or else the good news we see today could evaporate."