In preliminary figures for crimes reported to police, the bureau said the number of violent crimes declined by 1.4 percent from 2006, reversing two years of rising violent crime numbers. Violent crime had climbed 1.9 percent in 2006 and 2.3 percent in 2005, alarming federal and local officials.
Property crimes were down 2.1 percent last year from the previous year, the largest drop in the last four years.
Because the FBI preliminary figures do not contain the detailed breakdowns by age, race and gender of the final report that comes later in the year, one expert, James Alan Fox, said they may unintentionally mask a growing murder rate among black male teenagers and young adults, particularly with guns.
"We shouldn't be fooled into thinking our problems are over," said Fox, a criminal justice professor at Northeastern University. Fox said that from 2002 to 2006, the rate of murder committed by black male teens rose 52 percent.
"Violence is down among whites of all ages and both genders; it's up among black males, not black females," Fox said. "When you blend all the national numbers together you fail to see this divergence. There are many more whites in the population, so their decline can dwarf the increase among young black males."
Fox said black males are "feeling the impact of the economic decline and an increase in gangs and illegal gun markets. Gangs and youth crime are a growing problem despite these rosy statistics."
FBI spokesman Richard Kolko said, "One preliminary report does not make a trend, but it's going the way we want it to go." Kolko cautioned against putting too much significance on any shift that hasn't lasted at least two years.
Justice Department spokesman Peter Carr called the report "very encouraging" though he noted the final report could alter the figures.
"The report suggests that violent crime is decreasing and remains near historic low levels, which is a credit to increased cooperation among federal, state and local law enforcement," Carr said. "Some communities, however, continue to face localized violent crime challenges."
The largest declines were in vehicle theft, down 8.9 percent and in rape, down 4.3 percent and murder, down 2.7 percent.
The crime trends were not uniform. Murders, for instance, were down in cities of more than 250,000, including an enormous 9.8 percent drop in cities of more than a million residents. But murders rose in some small cities - up 3.7 percent in cities of 50,000 to 100,000, up 1.9 percent in cities of 100,000 to 250,000, and up 1.8 percent in cities under 10,000. Historically, murder trends have begun in the largest cities and moved over several years to smaller ones.
The other violent crimes tracked by FBI statistics - robbery and aggravated assault - were both down 1.2 percent.
The other property crimes in the FBI figures also declined, larceny-theft by 1.2 percent and burglary by 0.8 percent.
The FBI's preliminary crime report each year gives percentage changes rather than crime totals for national figures because not every jurisdiction has completed its reports yet.