WASHINGTON — Murders and robberies rose again across the country during the first six months of 2006, on pace for an increase in violent crime for a second straight year, preliminary FBI data released yesterday show.

The overall 3.7% uptick in violent crime between January and June comes amid a still-incomplete Justice Department study of 18 cities for clues on why criminal activity is increasing.

Property crimes like auto theft and other larcenies were down by 2.6% over the same six-month period, the data show. But the number of arsons shot up by nearly 7%, the FBI reported.

The numbers reflect what police across the country have been saying for months: that the lull in crime between 2001 and 2004 appears to be over.

“This is a concern we’ve been focused on,” said Gene Voegtlin, legislative counsel for the International Association of Chiefs of Police, which represents an estimated 20,000 law enforcement officials and has been pushing for more crime-fighting funding. Many police “agencies are really stretched thin when it comes to the budget and their ability to aggressively combat crime.” The Justice Department did not have an immediate comment. Last month, the department launched what Attorney General Alberto Gonzales called a study “to figure out the whys behind the numbers,” but Justice researchers have not yet visited all of the targeted regions, a spokesman said yesterday.

The early data show:

• Murders rose by 1.4 %, felony assaults by 1.2%, and robberies by a whopping 9.7% in 2006, compared to the first six months of 2005. The number of rapes decreased by less than one-tenth of 1%.

• Burglaries increased by 1.2%. But car thefts dropped by 2.3% and other stealing incidents by 3.8%.

• Arsons rose by 6.8%.

The data is based on crime reports from 11,535 police and other law enforcement agencies nationwide. The total number of actual crimes reported was not immediately available.