I disagree with the one recent post. Some of these "active shooters" may not have formal skill training or be ex-military/LE but their mental state & skill with video games(first person shooter) may aid their marksmanship.
As I posted in the past(before the awful Sandy Hook school shooting), there was a young boy who shot 15 of his 17 victims in the head. He later told investigators he never fired a real weapon but played a lot of video games.
I'd add that on YouTube.com is the office worker/labor response to a spree shooter/active shooter. It was produced by the city of Houston TX. It's called Run Hide Fight. I dont agree with all the points but it's a good way to open up or plan for any events in the workplace.
, has a few free online courses about active shooters & emergencies too. The video was made in 2011.
I'd add that I saw a YouTube.com video last night of author/instructor/match shooter Massad Ayoob discussing use of force training. He told the host about a big training seminar in 1995 of top level instructors/LE-spec ops/intel officers who used Simunition in a training drill. 120 out of the 130 were KILLED! Only Ayoob and 1 other member(a criminal psychologist) were able to eliminate the threat. Why? Because they were AWARE of their surroundings & watching out for danger!
As the US Army MP school cadre & staff always told us; Stay alert! Stay alive!