Nice leap. They absolutely should have known that this person was going to go kill a bunch of people because he killed one guy at a college. You know, because it's happened dozens of times in human history.
I love the American "mind." Start looking for someone to blame first. You're right. They should have mobilized their entire force to search what is a small city, ignoring the other (however many) millions of people they're responsible for over a much broader area. They should have stopped everyone from living their lives until they found the proverbial needle in the haystack. Nothing like ignoring reality.
At the end of the day, this was a single guy who did a really bad thing. I'm actually surprised, given how many idiots there obviously are, that stuff like this doesn't happen more often.