It can only happen where people are required to go unarmed and defenseless.
It can only happen where those with hate and a desire to kill the defenseless know that they can go and kill at will until the police come.
And as was demonstrated at Columbine when the police do come they will gather outside the building and wait and plan an entry strategy while they wait to determine if this is a hostage situation or a mass murder.
Colorado has some of the most liberal gun laws in the country.
They have a law called.....MAKE MY DAY........
You are permitted to shoot to kill anyone who threatens you OR your property.