Is this a hypothetical? "shoots up the bus" sounds rather extreme. If police receive a tip they know to be false, they are not obligated to "check it out." Problem is, most cops will not admit they knew a tip was false. In the hypothetical you describe, it will come down to why the officer chose not to frisk the suspect who then "shot up the bus."
Answered on Sep 27th, 2012 at 5:10 PM