Boston Marathon Bombing

On the FBI's radar: Shooter had been investigated before

He didn't come out of nowhere. The Orlando shooter came under suspicion three full years ago, after boasting of mutual acquaintances with the Boston Marathon bombers and making statements to co-workers that suggested he had radical, violent intentions. But after a 10-month investigation the FBI closed the case, finding no criminal charge to pursue.

Omar Mateen was scrutinized again in 2014 as part of a separate probe into a suicide bomber who attended the same Florida mosque and was a casual acquaintance. Again, agents found no significant radical ties and shifted their focus away.

Did investigators miss something they should have seen? A look at how law enforcement makes...