Why would it be a set up? Tunisia has had issues with ISIS and they are only next door to another stronghold in Libya and they did say they would try and get operatives on the boats crossing the med.
Did they manage to follow the correct person for the whole time? There would have been a lot of people running away from the scene in panic (just look at the videos after the attack in Nice) and it was at night so not at all possible they got people they were following mixed up.
At the end of...