What's really important here is incredibly sloppy counter-intelligence/police work. This was doomed from the start, because the motivation is "the big catch" and a desire to justify one's existence (TSA and DHS). Had anyone in the US government had even the slightest desire to do this right, things would have happened like this:
1. Disgruntled father-in-law sends email to FBI. Believe me, they get hundreds of these a week.
2. FBI says, "Is this real or is it just another crackpot?"
3. FBI contacts CIA, Swedish intelligence service, Interpol, etc, and says, "Do you have anything on this guy?"
4. Nothing turns up, FBI determines the email was from a crackpot, and the victim enters the US and is a welcome guest -- how we used to do things around here.
5. FBI's investigative work is inconclusive. Somebody says, "We think we might have a bad guy. Let's put a tail on him and maybe we'll find a bigger plot here at home." (i.e.: "The Big Catch.")
That's not how it works anymore. The motivation is to show the public and Congress how good you are at detaining and harassing people. That's how you justify your existence and how you keep getting money from Congress.
I look around every day and ask myself, "What happened to all the professionals?"