[mocha_nate]

My prediction: time traveler. Guy goes back in time to prevent an unspeakable tragedy that happened in the future. The simplest solution to alter the course of human history was this attack. We'll never find the killer because as soon as his work was completed, he vaporized into the ether as his timeline was culled.

[DougN7]

Wish that guy had … well, never mind. Better not to say it.

[IAmBroom]

I don't see how it plausibly could be anything else.