SPOILER ALERT; Soft Target, active shooters....
This topic mirrors a few plot elements in the novel; Soft Target by pro-2A author Steven Hunter(I Sniper, Point of Impact). If you read the great novel, you might get my point(s).
Active or "spree" shooters may have detailed plans or elaborate methods but if I were in Homeland Security or LE, I'd be leery of secondary IEDs or other terror acts designed to "draw in" security or LE.
A skilled marksman could shoot a target(citizen, police officer, elected official) then as the media & crowd grow, set off a IED or bomb.
More safeguards could be added too like alerts or "BOLOs"(be on the lookout) but many places have privacy/HIPPA type laws to prevent private details from public access.
Some states-cities like WA have "watch out for this dude" databases & the US Secret Service monitors people with; "inappropriate interests" towards federal level elected officials.