A 2013 tally by the National Law Enforcement Memorial Fund showed 100 officers died in the line of duty last year, the fewest since 1944. Traffic-related fatalities were the leading cause of officer deaths in 2013. The report found that "firearms-related fatalities reached a 126-year low ... with 31 officers shot and killed, the lowest since 1887 when 27 officers were shot and killed.Needless to say, the population of the United States and the number of police officers in the United States were far lower in 1887 than in 2013. Indeed, in 1887, the idea of a professional police force was only a few decades old anywhere in the world. On a per police officer basis, 2013 was the safest year to be a police officer ever.
The National Law Enforcement Memorial Fund methodology (which includes several more killings than the FBI data) is slightly more comprehensive than the FBI data which relies strictly on reports from other law enforcement agencies.
The story does not explain the trend, which includes a roughly one-third decline from 2011 to 2012, and a roughly 50% decline from 2012 to 2013, but wide use of bullet proof vests and improved trauma care, in addition to a reduced number of incidents overall, are important factors.