However Justice Department officials rejected any suggestion that federal funding cuts and a focus on anti-terrorism could be to blame.
The FBI said nearly 12,500 law enforcement agencies across the country reported an increase in the number of violent crimes of 2.5 per cent last year, led by more murders, robberies and aggravated assaults.
It said the number of murders rose 4.8 per cent, the largest percentage gain in 15 years. Robberies increased 4.5 per cent while the number of aggravated assaults went up 1.9 per cent. The only violent crime category to drop was rape, falling 1.9 per cent.
The nation's four regions all posted increases in violent crime in 2005. The Midwest had the steepest jump at 5.7 per cent, followed by the West at 1.9 per cent, the South at 1.8 per cent and the Northeast at 1.4 per cent.
Murders went up the most in cities with 50,000 to 500,000 people, rising between 9.4 and 12.5 per cent. The smallest increase in murders, just 0.5 per cent, occurred in cities with more than 1 million people.
While violent crimes went up, the number of property crime offences, like burglary, larceny-theft and motor vehicle theft, dropped 1.6 per cent.
The FBI report did not give any explanation for why the violent crime numbers and murders went up last year, but Justice Department officials said during a news briefing that the government's policies were not to blame.