WASHINGTON -- Violent crime rose across the country in the first six months of 2015 compared to the same period the year before, according to preliminary data released Tuesday by the FBI.

The statistics show a 1.7 percent jump in the number of violent crimes reported by local law enforcement to the FBI, with increases in murder, rape, robbery and aggravated assault.

Burglary and arson crimes, however, dropped in the first six months of last year, the FBI said.

Crime in the United States drew attention last year as many major American cities reported spikes in the number of homicides in their communities. The FBI statistics, pulled from 12,879 law-enforcement agencies, show a 6.2 percent jump in murders from January to June 2015.

"While the overall violent crime rates remain historically low, and it is too early to draw any long-term conclusions, the Justice Department is acutely focused on the increases being experienced in some communities of the country," said Justice Department spokesman Patrick Rodenbush.

He said the department next month would bring together representatives from every U.S. attorney's office for training on anti-violence strategies.

Police officials have struggled to pinpoint reasons for the homicide increase, and the FBI report does not offer explanations.

Many criminal-justice experts have cautioned against making too much of the increase, given the totals are far below where they were 20 years ago and noting after years of drops, the numbers could not be expected to keep going down.

FBI Director James Comey in October raised the possibility that officers, in the aftermath of the 2014 Ferguson shooting, had turned to less proactive tactics for fear of being captured on viral videos and attracting widespread condemnation. But the White House quickly distanced itself from that idea.

The statistics from the FBI offer only a limited snapshot since the data from local law enforcement agencies is reported to the FBI voluntarily. And there's no reason given for why crime might have increased.