Out of a clear blue sky, the first attacks on the World Trade Center unfolded
against a clear, crisp blue September sky, a clash of contrasts that
embodied the sense of strangeness that marked that day.

I was looking for a litteral interpretation of the expression "out of the blue" or "out of a clear blue sky" which describes an event that occurs unexpectedly, without any warning or preparation. There was not a lot of ways to put it, but although it is sad, this one just works.

At first, the airplane looks like a pinhole in the sky, but then it gets bigger and bigger until the fatal hit.

No such clear snapshot exists because nobody expected the first attack, and most people even thought of it as a dramatic accident. But then, the second attack proved that it was the result of terrorism. And of course all live cameras of the world recorded it.