In other incidents there have been active situations going on in a building and the cops would not enter until enough backup had arrived. I thought this was the usual method.
The debate is what should be done when there is an active shooter. After Columbine some Departments put out policies that in an active shooter situation, officers must engage the shooter. As FReeper the Original Dan Vik points out, while no one expects officers to commit suicide, they are expected to take risks if the needs of the incident require it. An officer needs to try to engage the shooter because the shooter is killing people who can NOT engage him. They are not ARMED.