Seventy-two law enforcement officers in the U.S. were slain in the line of duty in 2011, an increase of 16 over the previous year, according to an FBI report released Monday. The report does not include reasons for the rise.

The statistics collected by the FBI from police agencies across the nation reflects the varied nature of the killings. Fourteen of the victims were slain in unprovoked attacks. Eleven were killed while pursuing or stopping suspect vehicles. Six of the officers died while engaging in tactical operations. Five were slain in entrapments or ambushes, five others were killed while conducting investigations, and another five were killed after responding to robbery calls.

"Each of these losses is devastating," FBI Director Robert Mueller said in a statement, "and each one reminds us that the safety and freedom we enjoy come only at great cost."

The suspects used firearms in 63 of the 72 killings. That includes 50 by handguns. Seven were killed with rifles and six with shotguns.

The largest number of slain officers, by region, occurred in the South (29).

A final report including analysis of these killings, and the number of assaults on officers last year, will be released this fall in the annual uniform crime report.

The FBI figures were released at the beginning of Police Memorial Week. Many thousands of officers are expected to come to Washington to honor officers who lost their lives in the line of duty. Among the events scheduled Monday is a ceremony at the Bureau of Alcohol, Firearms, Tobacco and Explosives (ATF) Memorial Wall honoring William Henderson Foote, a Treasury officer who in was killed in the line of duty in Mississippi in 1883. Foote was the first African-American federal officer slain in the post-Reconstruction era.