It's not cops, it's bad witnesses.
Pretty much every single mass shooting (and even the recent Oregon Mall shooting) has had bogus reports of multiple shooters, with it turning out to be a lone gunman.
Well, the witnesses see the cops running around pointing guns, scaring the heck out of everyone and yelling and screaming well after the incident is over, they assume there must be something else going on.
It's just the cops trying to feel useful after they couldn't do anything AGAIN.