WASHINGTON — Both violent and property crimes declined in 2007 from the previous year, the FBI reported yesterday. But one expert warned the figures could mask rising murder rates among young black men.

In preliminary figures for crimes reported to police, the bureau said the number of violent crimes declined by 1.4% from 2006, reversing two years of rising violent crime numbers. Violent crime had climbed 1.9% in 2006 and 2.3% in 2005, alarming federal and local officials.

Property crimes were down 2.1% last year, the largest drop in the last four years.

The largest declines were in vehicle theft, down 8.9%, and in rape, down 4.3%, and murder, down 2.7%.

The crime trends were not uniform. Murders, for instance, were down in cities of more than 250,000, including an enormous 9.8% drop in cities of more than 1 million residents. But murders rose in some small cities — up 3.7% in cities of 50,000 to 100,000, up 1.9% in cities of 100,000 to 250,000, and up 1.8% in cities under 10,000. Historically, national murder trends have begun in the largest cities and moved over several years to smaller ones.

Because the FBI preliminary figures do not contain the detailed age, race, and gender breakdowns available in the final report later in the year, they may unintentionally mask a growing murder rate among black male teenagers and young adults, particularly with guns, a professor of criminal justice at Northeastern University, James Alan Fox, said.

“We shouldn’t be fooled into thinking our problems are over,” Mr. Fox said. He pointed out that between 2002 and 2006 the rate of murder committed by black male teens rose 52%.

“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.”