Still unclear, but it sounds like there was the bad guy with a knife, and a good guy with a gun. (Not a second bad-guy “shooter”).
And in another tweet it talked about seeing scissors and a big BBQ butane lighter on the ground. Guessing that was from a dumped tray of stuff that the TSA had removed from carry-on bags.
Sounds like a simple case of someone going bonkers for some reason while going through security. He goes crazy, whips out a knife and swipes some people and the TSA shoots him.