Violent crime in the U.S. rose slightly in 2006 for the second year in a row, according to a report released yesterday by the FBI.

The annual Uniform Crime Report, which this time includes a warning against using the figures to rank various cities and regions, shows that violent crime increased 1.9 percent from 2005 to last year.

While rape and aggravated assault dropped a bit, homicide rose 1.8 percent and robbery 7.2 percent.

Property crimes, meanwhile, fell 1.9 percent. The only increase was in burglary, which rose 1.3 percent over 2005.

The FBI compiles the UCR from information supplied by thousands of police departments.

The agency doesn't offer any explanations for patterns, but researchers caution that increases have to be considered in the context of historically low crime rates in the U.S.

The overall rate is still low, with 2004 and 2005 the lowest on record, but the 30-year drop seems to have leveled off.

"What we've seen in the last few years is a very flat national trend," said criminologist Alfred Blumstein of Carnegie Mellon University. "These are relatively minor fluctuations, except for robbery, and that's a cause for concern."

The crime picture here largely mirrored the national one.

Murders remained about the same in the city of Pittsburgh as in most years, at 56, according to the UCR.

But police here reported an increase in robberies. In Pittsburgh, for example, robbery rose 6.5 percent in 2006, although detectives say robberies have been down in 2007. Every year, researchers and the media use the UCR to rank states and cities.

This year, however, the FBI said that's probably not a good idea because rankings don't account for the complexities of crime in such a large and diverse nation.

"These rough rankings provide no insight into the numerous variables that mold crime in a particular town, city, county, state or region," the agency said in a release. "Consequently, they lead to simplistic and/or incomplete analyses that often create misleading perceptions adversely affecting communities and their residents."

The FBI listed a number of factors that influence crime patterns from place to place, including population density, urbanization, youth concentration and stability of the local population, cultural differences, education levels and effectiveness of local law enforcement.

That's why raw numbers can be deceiving. Professor Blumstein and other experts point out, for instance, that nationwide homicide trends can be difficult to discern because some cities, such as Baltimore and Detroit, have very high rates while other similarly sized cities don't.

Rather than focus on sheer numbers of crimes, criminologists prefer to look at the rate per 100,000 for a more complete picture.

According to the UCR, that rate crept up one1 percent over 2005.

But that figure also follows decades of falling rates. Since 1997, for example, the violent crime rate in the U.S. has plunged 22.5 percent.

In Pennsylvania, police reported increases consistent with the national figures for some crimes but not others.

Burglary statewide rose 1.8 percent and robbery jumped 8.3 percent, yet murder fell 5 percent from 759 to 721.

The increase in robbery seems to be a common denominator across the country.

The national rate had consistently fallen, with one blip in 2001, since the peak of 272 per 100,000 in 1991.

It bottomed out at 136 in 2004, and now has climbed for two years straight to about 149.

No one seems to know why.

"I really don't have an explanation, not offhand," said Cmdr. Tom Stangrecki, head of major crimes for the Pittsburgh police. "I'd have to collect all the data from various sources."

In the city, zone officers handle most street robberies, while detectives deal with business holdups, home invasions or holdups in which someone gets shot. Cmdr. Stangrecki said he'd have to examine the entire case load before he could draw any conclusions.

Professor Blumstein also had few answers except to say that the robbery spike could be driven by those with "low skills in an economy that is increasingly demanding."

Regardless of the reason, the increase in robbery this year continues a five-year trend.

For the UCR, police keep track of four kinds: armed with a gun, armed with a knife, armed with another weapon and unarmed.

The largest percentage increase among those four was robbery with a gun, up 9.9 percent over 2005.

In Pittsburgh, robbery incidents rose from 1,617 in 2005 to 1,722 last year.

Officers here have noticed more juveniles carrying out robberies using BB guns that look like the real thing, but beyond that police could offer no insight into patterns.

The two-year increase in violent crime nationwide after so many years of decline has some in law enforcement concerned that a new crime wave is coming.

The Justice Department has been trying to head it off through such efforts as increased federal prosecutions of gun offenders and task forces to combat gangs in metro areas struggling with surges in violence.

Among the cities cited by former Attorney General Alberto Gonzales earlier this year were Cleveland, Charlotte, Memphis and Sacramento.