Attorney General Eric Holder has called for more training for police who respond to active shooter incidents, which he says are growing in number and becoming more deadly. NBC's Pete Williams reports.

Attorney General Eric Holder says the number and the lethal nature of active-shooter incidents nationwide have soared over the past five years.

In remarks to the nation's police chiefs in Philadelphia on Monday, Holder said the United States saw an average of five active shooting incidents a year between 2000 and 2008.

"Alarmingly, since 2009, this annual average has tripled. We've seen at least 12 active shooter situations so far in 2013."

The Department of Homeland Security defines an active shooter as an individual actively engaging in killing or attempting to kill in a confined and populated area. Recent examples include the Sandy Hook Elementary School shooting in Newtown, Conn., in December and the Navy Yard shooting in Washington, D.C., last month.

Over the past four years, Holder said, the number of people shot and killed in these incidents is up nearly 150 percent.

"It's become clear that new strategies and aggressive national response protocols must be employed to stop shooters in their tracks," Holder said.

Many police departments have abandoned the contain-and-wait strategy in favor of a more aggressive response that calls for the first officers on the scene to attempt to engage the shooter instead of waiting for the SWAT team to arrive.

"That's why all law enforcement officers must have the best equipment and most up-to-date training to confront these situations," Holder said. "We owe these officers nothing less."

Holder said the Justice Department, over the past decade, has helped train 50,000 front-line officers, more than 7,000 on-scene commanders, and over 3,000 local, state, and federal agency heads on how to respond to active shooters.