This question can be answered using data provided by the Federal Bureau of Investigation in its Uniform Crime Report (UCR). The relevant part of the UCR can be found at this link.
In 2010, 128 law enforcement officers died in the line of duty in the United States and Puerto Rico. Of these 128, 52 were feloniously killed. The other 76 died from accidents suffered while on duty. One or more officers was feloniously killed in each of 22 states and Puerto Rico.
The link above provides access to more information about the officers killed and the circumstances in which they were killed.