Mateen was a 'known quantity' to federal law enforcement before he killed 53 people in the worst mass shooting in U.S. history. Omar Mateen of Port Saint Lucie, Florida, came to the attention of federal authorities twice prior to being identified as the gunman in the Orlando nightclub mass shooting, a senior law enforcement source told The Daily Beast. [Snip] The senior law enforcement source reports that Mateen became a person of interest in 2013 and again in 2014. The Federal Bureau of Investigation at one point opened an investigation on Mateen but subsequently closed the case when it produced...