Fox News reported on Arizona Border Patrol Agents who are questioning DHS procedures regarding an Active Shooter. Apparently the procedures apply to not only civilians but armed law enforcement as well.
The summary of the procedure is when encountering an active shooter, individuals should try to run away and hide. If that is not an option, then throw objects at the active shooter. Only as a last resort should armed law enforcement use lethal force.
Here is the link to the FEMA-adminstered computer course, IS-907- Active Shooter: What You Can Do.