FBI data show that the number of law enforcement officers killed in homicides has steadily been declining since the early 1970's.
Source: Homicide trends in the U.S.: Law enforcement officers killed, US Dept. of Justice, Bureau of Justice Statistics
Trend: Homicides decreasing, accidental deaths increasing
Meanwhile, in the last decade, the number of accidental deaths have been increasing. In the ten years from 1998 to 2007, the number of homicides exceeded the 16-year average only once, while the number of accidents exceeded the average seven times. From 1998 onwards, accidental deaths exceeded homicides.
Source: US Federal Bureau of Investigation (FBI), Criminal Justice Information Services Division