U.S. Attorney General Eric Holder told law enforcement leaders that patrol officers should be the nation's first line of defense against active shooters while speaking at the IACP Conference in Philadelphia.

In his Monday speech, Holder urged patrol officers to take an "immediate, aggressive response" to a gunman seeking targets at a school, workplace, or other public area.

Patrol officers are "the tip of the spear" in law enforcement's response to active shooters and Holder said the U.S. Department of Justice has helped train 50,000 front-line officers and more than 3,000 local, state, and federal agency heads.

"To save lives, the first officers who arrive must be the ones to confront these active shooters," Holder said.

Holder said between 2000 and 2008 the country saw an average of five active shooters per year. Since 2009, that average has spiked, including 12 active shooter attacks so far in 2013, Holder said.