There is a part of the story out there that has Omar traveling privately to Afghanistan. How does one return to the US with that stamped on your Passport?
To me if the country is going to debate gun controls perhaps the debate should center on giving away US citizenship to potential terrorists.
It seems his twin trips should have been enough to raise suspicions:
http://www.foxnews.com/us/2016/06/15/mateens-twin-trips-to-saudi-arabia-raise-suspicions.html