Falling crime rates have been the norm in the United States for two decades, but newly released data raise the question of whether the trend is reversing itself. Statistics released by the Bureau of Justice Statistics show that violent crime has risen for two years in a row.

The violent crime rate rose 15.5 percent from 2011, the bureau reported, from 22.6 victimizations per 1,000 persons to 26.1. That rate includes sexual assault, robbery, and aggravated and simple assault.

The data come from the National Crime Victimization Survey, a snapshot of U.S. crime trends based on reports from a representative sample of Americans. Most of the increase came in crime not reported to the police and simple assault, an attack that does not result in serious injury. Nearly one in six crimes went unreported in 2012.

What the reports mean, said Dennis Kenney, a professor at the John Jay College of Criminal Justice, is that “we’ve hit the bottom of the decline” in crime, “and are starting to go back up.”

The most worrying aspect of the report might be a rise in self-reported property crime that did not appear in statistics reported by police. The incidence of household burglary, theft and motor vehicle theft increased 12 percent from 138.7 per 1,000 households in 2011 to 155.8 in 2012.

That could be a result of increasing economic insecurity: People file a police report only when the loss of property exceeds their insurance deductible, and many people have raised deductibles to save money.

It also could reflect the growing popularity of mobile technology such as iPhones. “With growing inequality gaps, there are more people who covet” such items, said Urban Institute crime researcher John Roman.

It also could be caused by a failing trust in police, Kenney said, noting that other countries with lower levels of social integration have higher rates of unreported crimes.

It's not clear, however, what might be causing the rise in crime, or whether it will keep rising. Kenney attributed it to "a general coarsening of the social contract."

Others aren't so sure. “Only two years' worth of data isn't sufficient to say anything about major factors,” said John MacDonald, chairman of the Department of Criminology at the University of Pennsylvania, noting that the low base rate of victimization limits what can be inferred from percentage changes in the report.

Violent crime has fallen by roughly 75 percent over the past two decades, as measured by NCVS data. From 1992 to 2011, according to separate statistics from the FBI, the murder rate fell by roughly half, and forcible rapes dropped nearly 40 percent.

The FBI data, reported by police, better capture long-term trends, Roman said. He downplayed the new data, noting that violent crime has fallen in 19 of the past 21 years.

But Kenney warned the report could be the first sign of trouble.

The increase “in itself is not as significant as the trend,” Kenney said, noting that crime trends can be difficult to reverse. “It may be difficult to put the brakes on and stop it from going up," he cautioned, noting that it took both an overhaul in justice policy and a change in U.S. culture to reduce crime rates.