Confirmation bias is an enemy of accurate threat assessment. It is simply paying attention to evidence that supports your belief about something and ignoring evidence that refutes it. Let the investigation drive your theory, not the other way around.
If you are planning for an active shooter, you will miss the active knifer (about 40% of school attackers use edged weapons).
If you believe the attacker will be a stranger, you will miss the current of former student (about 91% of the attackers in schools) or the current or former employee (about 78% of the attackers in workplaces).