How Can it Be Cracked?

Given my very limited understanding of engineering, I will grant you that I don't know what the hell I'm talking about.

There is a crack in the Washington Monument, and yet, I thought the monument was assembled by stacked pieces of granite and that no mortar was used. If there is a crack, then does that mean that the original granite bricks have cracked or that there is a separation between the pieces (which, as I said, are not connected by mortar)?

I hope they can get it repaired soon.