WASHINGTON — Murders, rapes and other violent crimes dropped sharply in the United States in the first six months of 2011, continuing a downward trend that has lasted 4 1/2 years, the FBI reported on Monday.

The federal law enforcement agency said preliminary January-through-June figures showed the number of violent crimes declined 6.4 percent from the previous year, led by a 5.7 percent drop in murders and a 5.1 percent decrease in rapes.

In other violent crime categories, robberies declined 7.7 percent while aggravated assaults fell 5.9 percent.

The FBI’s regular statistical report did not give any reasons for the lower crimes nationwide. But the latest numbers provided further evidence of no crime spike coinciding with the tough economic conditions and high unemployment.

The report is based on information from more than 12,500 police and other law enforcement agencies across the country.

Violent crimes went down about the same amount in all four regions of the country and decreased in big cities, smaller cities and rural areas.

Property crimes, such as burglary, larceny, theft and motor vehicle theft, also declined in the first half of the year, with burglaries down 2.2 percent, larceny and theft down 4.0 percent and stolen vehicles down 5.0 percent.

The separate arson category declined 8.6 percent in the first six months of the year, the FBI said.