It was the worst mass shooting in U.S. history, and now we know exactly how it happened.
[Image via AP Images.]
How do you bring down the house at the X Factor finale with just five guys?
Make them all go One Direction!
Ch-ch-check out the video (above) to see 1D rock out with their Midnight Memories out!
And be sure to look for Demi Lovato singing along! She's totally a Directioner!