Although violent crimes have decreased overall in the United States, government numbers show the number of attacks against police officers have tripled over the past six years.

The Federal Bureau of Investigation found that 72 police officers were killed by suspects in 2011 -- 25 percent more han the previous year, and a 75 percent increase since 2008.

"We haven't seen a period of this type of violence in a long time,"Commissioner Raymond W. Kelly of the New York Police Department said.

The majority of the deaths occurred in smaller cities.

Some officials believe the increase is due to new techniques embraced by some departments, where officers are encouraged to conduct more street stops and to seek out and become more confrontational with suspects.

In many of the reported deaths, the officers were trying to arrest a suspect who had a previous record of violent crime.