To those interested in the 20 page report itself, Homeland Security Digital Library, US Secret Service "Mass Attacks in Public Spaces - 2018" [open pdf - 3MB]:
https://www.hsdl.org/?view&did=826876
In addition there are the FBI Active Shooter Incident reports (public shooting sprees where police or bystanders were in a position to intervene):
Schweit, Katherine W., "Active Shooter Incidents in the United States in 2014 and 2015", Federal Bureau of Investigation, 2016
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https://www.fbi.gov/file-repository/activeshooterincidentsus_2014-2015.pdf
"Active Shooter Incidents in the United States in 2016 and 2017", Federal Bureau of Investigation, April 2018.
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https://www.fbi.gov/file-repository/active-shooter-incidents-us-2016-2017.pdf
FBI Active Shooter Incident reports 2014-2015 and 2016-2017 note that of a total 90 incidents, 14 active shooters were stopped by bystanders who were not police. The FBI 2016-2017 report stated: "Their selfless actions likely saved many lives."
In the fourteen successful interventions:
7 unarmed citizens confronted, restrained, blocked, or talked the shooter into surrender.
1 citizen with pepper spray restrained the shooter.
4 citizens with firearms permits exchanged gunfire with the shooter.
2 citizens with firearms permits detained the shooter at gunpoint for arrest by responding police.
In three other incidents, citizens with firearms permits intervened without success:
1 was killed.
1 was wounded.
1 forced the shooter to flee the original crime scene.
I find no incident in the FBI reports of responding police shooting a citizen who was acting in defense of self or others or who was detaining the spree shooter.
Of active shooter incidents meeting the FBI definition, citizens with firearms permits intervened in 9 incidents, 1 out of 10.
ADDED:
Apples, and Oranges, and Pears, Oh My.
Mass Attack in a Public Space, to the USSS, is defined as an incident "during which three or more persons were harmed", not necessarily killed, carried out in a public place. "[W]e included acts of intentional violence in public (e.g., parks, community events, retail establishments) or semi-public (e.g., workplaces, schools, religious establishments) places during which significant harm was caused to three or more persons [regardless of weapon(s) used]. We excluded violence related to criminal acts (e.g., gang or drug activity), failed attempts at a mass attack, or spontaneous group violence [riot, flash mob, etc]."
Active Shooter Incident, to the FBI, is defined as "a situation in which a shooting is in progress" and "both law enforcement personnel and citizens have the potential to affect the outcome of the event based upon their responses", regardless of body count.
Mass Killing, to federal law enforcement, is defined as three or more (not counting the killer) dead from any cause. Mass killings are not solely mass shootings.
For 2016-2017, 40% of Active Shooter Incidents also met the definition of Mass Killing.
United States Secret Service (USSS) Mass Attack in a Public Space definition appears to include FBI Active Shooter Incident.
Active Shooter Incident (a shooting in public where police or bystanders could intervene) may have no one harmed or killed, or may meet Mass Attack in a Public Space if three or more harmed, or meet Mass Killing if three or more are killed.
But, FBI Active Shooter Incident and USSS Mass Attack in a Public Space
exclude crimes in private places without witnesses other than perpetrator(s) and victim(s), such as the typical gang violence or most assaults or homicides, which usually occur when and where there is no chance of response by law enforcement or bystanders.