WASHINGTON — Violent crime dropped 6 percent in 2010, marking the fourth straight year-to-year decline, while property crime was down for the eighth straight year, falling 2.7 percent, the FBI reported Monday.

Nationwide, there were an estimated 1.2 million violent crimes reported to authorities in 2010 and an estimated 9 million property crimes.

An aging population, better policing and continued high rates of imprisonment for criminals are helping to drive down crime rates, criminologists say.

Robbery fell 10 percent, rape dropped 5 percent, and murder, non-negligent manslaughter and aggravated assault fell more than 4 percent.

Each category of property crime offenses decreased in 2010. The largest decline, 7.4 percent, was for motor vehicle thefts. Burglaries decreased 2 percent and larceny-thefts declined 2.4 percent.

“The last thing we should do is get complacent and say ‘mission accomplished,’ and so let’s transfer resources away to other areas,” said James Alan Fox, a criminologist at Northeastern University. “You don’t solve the crime problem. You only control it. Without sufficient support for policing and crime prevention, the numbers can go back up.”

The expectation had been that crime would increase in a weak economy with high unemployment like that seen in 2010.

“The connection between crime and the economy is an illusion,” said University of Cincinnati professor John Eck, who teaches criminal justice research methods. Eck suggested that one important factor in driving down crime rates is improved policing practices that focus on high-crime locations in which “you are denying the criminal the ability to attack targets. Virtually every large city is doing something.”

Eck said since the 1970s, property crime has been going down and “probably the best explanation is that people are getting better at protecting their belongings, with all the simple devices industry has made to help — from car burglar alarms to better bicycle locks.”

Last week, the U.S. Bureau of Justice Statistics reported that the number of violent crimes fell by a surprising 12 percent in 2010 and that there were 3.8 million violent crimes last year.

The BJS survey of households includes crimes both reported and not reported to law enforcement. In contrast to the household survey, the FBI report includes homicide, arson, commercial crimes and crimes against children under age 12. The FBI report captures only crimes reported to law enforcement, while excluding sexual assaults and simple assaults.