This is not about how to end mass shootings, well maybe it is, this is the best information we have about how mass shootings end.

This is a section from an FBI report entitled "A Study of Active Shooter Incidents in the United States Between 2000 and 2013" where they looked at all the active shooter incidents during that period. Note it is not a study of "mass shootings" the FBI does not use that term.

Resolutions

The majority of the 160 incidents (90 [56.3%]) ended on the shooter’s initiative— sometimes when the shooter committed suicide or stopped shooting, and other times when the shooter fled the scene.

There were at least 25 incidents where the shooter fled the scene before police arrived. In 4 additional incidents, at least 5 shooters fled the scene and were still at large at the time the study results were released.

In other incidents, it was a combination of actions by citizens and/or law enforcement that ended the shootings. In at least 65 (40.6%) of the 160 incidents, citizen engagement or the shooter committing suicide ended the shooting at the scene before law enforcement arrived.
Of those:
■ In 37 incidents (23.1%), the shooter committed suicide at the scene before police arrived.
■ In 21 incidents (13.1%), the situation ended after unarmed citizens safely and successfully restrained the shooter. In 2 of those incidents,24 3 off-duty law enforcement officers were present and assisted.
■■ Of note, 11 of the incidents involved unarmed principals, teachers, other school staff and students who confronted shooters to end the threat (9 of those shooters were students).
■ In 5 incidents (3.1%), the shooting ended after armed individuals who were not law enforcement personnel exchanged gunfire with the shooters. In these incidents, 3 shooters were killed, 1 was wounded, and 1 committed suicide.
■■ The individuals involved in these shootings included a citizen with a valid firearms permit and armed security guards at a church, an airline counter, a federally managed museum, and a school board meeting.25
■ In 2 incidents (1.3%), 2 armed, off-duty police officers engaged the shooters, resulting in the death of the shooters. In 1 of those incidents, the off-duty officer assisted a responding officer to end the threat.26
Even when law enforcement arrived quickly, many times the shooter still chose to end his life. In 17 (10.6%) of the 160 incidents, the shooter committed suicide at the scene after law enforcement arrived but before officers could act.
In 45 (28.1%) of the 160 incidents, law enforcement and the shooter exchanged gunfire. Of those 45 incidents, the shooter was killed at the scene in 21, killed at another location in 4, wounded in 9, committed suicide in 9, and surrendered in 2.

24 Santana High School, March 5, 2001; Appalachian School of Law, January 16, 2002. (See Appendix A).
25 Player’s Bar and Grill, May 25, 2008; Tom Bradley International Terminal at Los Angeles International Airport, July 4, 2002; Youth with a Mission Training Center/New Life Church, December 9, 2007; United States Holocaust Memorial Museum, June 10, 2009; Panama City School Board Meeting, December 14, 2010, Shooter wounded.
26 AT&T Wireless, May 27, 2010; Trolley Square Mall, February 12, 2007