Crime continues to fall in all major categories, a streak that has now hit 4½ years, according to data released Monday, but killings of police officers are on the rise this year.

Violent crime fell 6.4% in the first half of 2011, compared with the same period last year, with declines seen in every region of the country, Federal Bureau of Investigation figures showed. The preliminary data were gathered from about 12,500 local law-enforcement agencies.

Police...