They were trying to get him earlier, but due process stopped law enforcement from acting. As soon as he violated his Visa requirements, they moved in. The takeaway I had from this, is that they're claiming to have stopped a mass casualty event by preemptive action. If they weren't limited by due process and acted preemptively on a regular basis, would they pat themselves on the back every time for preventing something that rarely happens? This is like a lame version of Minority Report.