[UPDATE: There is a recent FBI study on this subject with somewhat different figures but the new study appears to have major flaws.]
Of the “active shooters” who are stopped either DURING or BEFORE commission of murder, only 25% are stopped by police officers. Another 25% are stopped by ARMED CIVILIANS. And still another 50% are stopped by UN-ARMED CIVILIANS.
The statistics do not include the active shooters who complete a murder spree and then commit suicide BEFORE they are confronted. The statistics only include those shooters who ultimately have been STOPPED (this includes shooters committing suicide when confronted by armed civilians or police) or KILLED during a confrontation with either police or civilians. It also includes active shooters who have been stopped but have not actually committed mass murder.
This information was gathered by former SWAT team officer, and current police training school owner, Ron Borsch of the S.E.A.L. Academy. It is now considered valid by the L.E.O. trainers and many departments have changed their tactics based on Mr Borsch’s work so that the first officers on the scene no longer wait for back up but now go into the scene to actively search for the shooter.
Also note from DHS (http://www.dhs.gov/xlibrary/assets/active_shooter_booklet.pdf):
Because active shooter situations are often over within 10 to 15 minutes, before law enforcement arrives on the scene, individuals must be prepared both mentally and physically to deal with an active shooter situation.
FREE CONCEALED CARRY CLASS: www.guntrust.org
Also see: Massad Ayoob: http://backwoodshome.com/blogs/MassadAyoob/2009/04/24/among-the-cops-and-their-instructors/