During a scuffle, Boston's Shawn Thornton approached Pittsburgh's Brooks Orpik from behind, dragged him down to the ice and repeatedly punched him in the face as he lay on his back, seemingly helpless on the ice.
The game had been chippy from the start. That scuffle started after the Penguins' James Neal appeared to injure the Bruins' Brad Marchand with a knee to the head.
In a different scrum after that, Thornton went after Orpik, who appeared helpless to fight back. Orpik had leveled Loui Eriksson with a heavy check earlier in the game, sending the Bruins forward sprawling to the ice.
According to NESN, Eriksson took a few seconds to compose himself on the ice, then headed to the locker room. The veteran winger missed five games due to a concussion earlier in the season when he took an illegal hit to the head from Buffalo Sabres forward John Scott.
During the scrum in question, Orpik was speaking to the officials and Thornton skated over to the defenseman. Then this happened:
Orpik remained on the ice for a significant amount of time before being placed on a stretcher and taken off the ice.
Orpik was taken to Massachusetts General Hospital, but the Penguins reported that he was released and would be going home with the team.
Orpik has returned from the hospital and will be traveling with the team back to Pittsburgh, per #Pens coach Bylsma.
Penguins coach Dan Bylsma said Orpik has a concussion.
"I think he was out on the ice for a period of time," Bylsma said. "So I think that means he's got a concussion, for sure."
Thornton was given a match penalty for deliberate attempt to injure.
He will likely be looking at a disciplinary hearing with the NHL front offices and is almost certain to face a suspension.
FOX Sports NHL analyst Chris Chelios said: "If history repeats itself, the league will justify whatever decision they want, fair or not. There is no consistency in suspensions and always double standards with the players involved."
Thornton was apologetic after the game, saying he "felt sick" about the incident.
"Obviously, I made a mistake. I’m aware of it. ... It's hard for me to talk about it right now," Thornton said. "I can't say I'm sorry enough."