Check this 2015 report from the Orlando Sentinel:
During the Emergency Nurses Association’s annual meeting on Friday, a mock shooter blew himself up, killing and injuring 100 local volunteer actors.
There was blood, screams and moans, wounds and dead people – all fake – to create a realistic training environment for more than 40 nurses who had signed up for the exercise.
They quickly began tending to “patients,” assessing the extent of their injuries.
The nurses association had worked with several Orlando-based companies for nearly a year to set up the drill on at the Orange County Convention Center, which by coincidence took place a day after the shooting at a community college in Roseburg, Oregon, where 10 people died and seven were wounded.
Seems there is always some kind of “shooter drill” or “terror simulation” going in the same place that a supposed “real” event later takes place.