The incident happened late in the first period of the Pittsburgh Penguins' 3-1 win over Florida.
Mitchell grabbed Letang's helmet off his head and smacked him with it as referees tried to separate the two in a melee near the Panthers net.
The melee started when Sidney Crosby deked through Mitchell and tried to stuff the puck between Roberto Luongo’s pads as he skated past. Luongo stopped the first poke, but Kris Letang followed up with a jab to try to push it in.
That touched off a scrum in the corner that saw the Panthers captain go after Letang for poking his goalie. Crosby and Sean Bergenheim started wrestling, and Mitchell dragged Letang out of the scrum and tried to lay into him.
The refs tried to break it up, but Mitchell got his left arm free and landed a few blows before inexplicably seizing Letang's helmet. He took the helmet in his left hand and gave it a good swing, catching Letang on the side of the face.
It looked like a heat-of-the-moment move and it's hard to picture the league issuing a suspension, but they may fine Mitchell for the incident. He used the helmet like a weapon, and the last thing the league wants to see is its players doing something like that.
Both Letang and Mitchell received fighting majors, and the refs handed out a total of 18 penalty minutes in the scrum.