WASHINGTON - The number of violent crimes fell by a surprising 12 percent in the United States last year, a far bigger drop than the nation has been averaging since 2001, the Justice Department said.

The Bureau of Justice Statistics reported there were 3.8 million violent crimes last year, down from 4.3 million in 2009.

Experts aren't sure why. The expectation had been that crime would increase in a weak economy with high unemployment like that seen in 2010.

The reality is that "we're surprised to find how much it declines," Professor Alfred Blumstein of Carnegie Mellon University's Heinz School said Friday.

The big drop dwarfs the 3 percent yearly decline in violent crimes the nation averaged from 2001 through 2009.

More than 80 percent of the decline in violent crime was attributed to a plunge in simple assaults, by 15 percent. Those assaults accounted for nearly two-thirds of all violent crimes in 2010.

The combined total of property crimes and violent crimes was down 6.6 percent last year, from 20 million to 18.7 million.

The numbers come from the National Crime Victimization Survey, which gathers information on nonfatal crimes against people aged 12 or older by questioning a nationally representative sample of U.S. households.

Turning to rates of crime per thousand residents, which takes into account population growth over time, it's clear that the decline in violent crime is part of a long-term trend that began in 1993.

From 1993 through 2010, the rate of violent crime has declined by a whopping 70 percent: from 49.9 violent crimes per 1,000 persons age 12 or older to only 14.9 per 1,000 in 2010.

Half of this decline came between 1993 and 2001. Between 2001 and 2009, violent crime declined at a more modest annual average of 4 percent, but that rate decline jumped to 13 percent in 2010.

From 2001 through 2010, the rate of property crime fell by 28 percent.

The rate for violent crime is based on the number per thousand population. The rate for property crime is based on the number per thousand households.

Blumstein added that "the victimization survey is basically confirming" the FBI's preliminary figures from last May on crimes reported to police during 2010. That early, incomplete FBI data showed reported crime fell across the board last year, extending a multi-year downward trend with a 5.5 percent drop in the number of violent crimes in 2010 and a 2.8 percent decline in the number of property crimes. The FBI's final figures for last year will be released Monday.

The victimization survey figures are considered the government's most reliable crime statistics, because they count crimes that are reported to the police as well as those which go unreported. Over the last decade, the government has found that only about half of all violent crimes and only 40 percent of property crimes are reported to police.

Because the survey is based on interviews with victims, it gathers no data on murder. But the FBI's crime figures, based solely on what is reported to police, do provide murder figures, and they are considered quite reliable because murder has always been the least likely crime to go unreported. Murder is by far the least frequent major crime, only 15,241 in 2009.