In that case, what I don't get is why it hasn't happened repeatedly. You don't need to go to a training camp to kill tens or hundreds of people. Imagine two or three people at different points in a crowd, each with a homemade explosive or even a simple few guns. One blows up/opens fire, the crowds panic, another one blows up/opens fire, people panic even more, people trampled to death, hundreds dead in any busy area. It's not something you need to learn, it'd not something you could gather intelligence on. You could decide to kill a few people one afternoon and be done by that evening.