"The reality is that police don't always have the luxury of time to get their most highly trained, best-equipped officers on the scene," he said. "To save lives, the first officers to arrive must sometimes be the ones to directly engage an active shooter. That's why all law enforcement must have the best equipment and most up-to-date training to confront these situations."
Holder's remarks and Monday's violence in Nevada came nearly a year after a gunman shocked the nation by opening fire inside Sandy Hook Elementary School in Newtown, Conn., killing 26 and igniting a new round of national gun-control debates.
The incident at the Sparks middle school, east of Reno, was not nearly as deadly. But local police described it as just as chaotic.
The gunman, whose name has not been released, was found dead minutes after the shooting began. It remained unclear Monday evening whether he committed suicide or was shot by someone else, police said.
Holder said Monday that the annual number of mass-shooting incidents nationwide has tripled over the last four years, from an average of five a year before 2009 to about 15 a year since.
Before Monday, authorities had responded to 12 such incidents across the country this year, including a shooting rampage last month inside the Washington Navy Yard that left 12 dead and injured three others.
Since the Newtown incident late last year, Holder has became increasingly critical of the nation's gun-control laws. His office has also trained 50,000 frontline officers and more than 3,000 local agency heads on how to respond to shooters, he said Monday.
But despite his pledge of continued support for local law enforcement, not all at Monday's conference welcomed the attorney general's pledges. At the back of the hall, a handful of uniformed officers could be heard grumbling, "Three more years of this?"
And Craig T. Steckler, president of the police officers' conference, led his introduction of Holder by criticizing the Justice Department's decision this year not to challenge state laws in Colorado and Washington that allow recreational marijuana use.
"We have entered a slippery slope that will be hard to turn away from," Steckler said, adding that his organization's relationship to the Justice Department often worked like a contentious marriage.
Holder did not respond directly to Steckler's drug-policy complaints during his speech Monday except to quip: "We may need marriage counseling."
This article contains information from the Associated Press.