https://www.thedailybeast.com/articles/2016/06/12/omar-mateen-id-d-as-orlando-killer.html
I misspoke. They don't say he's been on their watch list, but they say he was investigated in 2013 and 2014 and “He’s been on the radar before.”
For unrelated crimes. Nonetheless, this is an interesting development as it reminds us that anyone can become radicalized. Europe has a bigger terror problem than the US because the massive number of alienated Muslim youths who become radical because they want to belong to something. But as long as violent interpretations of religion exists, we can never have perfect security.