That's pretty much always been the case with most of these attacks. Which is the very same reason why I think that "more surveillance/data collection" will make the problem only worse.

As is, many government agencies databases seem to be filled to the brim with false-positives, making it impossible to spot the actual dangerous people among the hundreds of thousands (if not millions) of "suspects".

Maybe they believe that more data will make it easier to figure out who the really dangerous people are? But that whole idea is still based on concepts which pretty much boil down to precognition of how individual humans gonna act, an impossibility.

Guess a few people took Minority Report a tad bit too seriously and didn't get the message at all.

If you are having trouble finding a needle in a haystack, adding more hay is a bad idea.

source: https://www.youtube.com/watch?v=V9_PjdU3Mpo

That's a good one, reminded me of this: https://www.youtube.com/watch?v=pdIA0jeW-24

They were probably on watch lists at least in part due to the ease of monitoring their communications.