I'm just a little concerned that this even needs to be said;
We don't preemptively act against people in this country for what they "might" do - unless it's a form of conspiracy. Other countries do that, but they're not real safe either.
Cases against suspicious people and/or activities are opened and investigated on a continual basis by Federal, State & Local agencies. If no crime has been committed, then there's nothing to do preemptively but wait and watch. If the risk is high value, then someone is assigned to do some level of monitoring, but there's a limit to how much of that can be done. There aren't enough LEO's on the planet to "bird dog" every knucklehead that has some potential to be a violent.
Lastly, activities like beating your wife, being an asshole to your coworkers, talking trash about American values, public policies, or expressing religious intolerance is as common as dirt.
Assigning risk to guys like Mateen would be like trying to ID which mosquitos are carrying the Zika virus.