Violent crimes dropped sharply in the United States in the first six months of 2011, continuing a downward trend that has lasted more than four years, the Federal Bureau of Investigation (FBI) reported Monday. Property crimes also declined. Preliminary figures showed the number of violent crimes dropped 6.4 percent from the previous year, led by a 5.7 percent decrease in murders and a 5.1 percent decline in rapes. In other violent-crime categories, robberies fell 7.7 percent while aggravated assaults dropped 5.9 percent. Property crimes, like burglary, larceny, theft, and motor-vehicle theft, also declined in the first half of this year, with burglaries falling 2.2 percent, larceny and theft dropping 4 percent, and stolen vehicles decreasing 5 percent. The separate category for arson fell 8.6 percent.