They had the suspect in custody when the shooting started. He was a former boyfriend of the girl who was killed and they arrested him outside the middle school that my kids attend. When the shooting broke out, they realized they had the wrong guy. Note: At the time, he was arrested, a kid was trying to sell pistols out of a backpack in that same middle school.
Then I think you mean that they had "a" suspect in custody, not necessarily "the" suspect. If an alarm system had existed, I doubt that it would have been triggered.
The liberals continue to seek some way to stop an armed killer from killing people that does not involve having the threatened people fight back effectively. I can't figure out how any creature with testosterone in their system can think this way.