The National Hockey League is facing a growing backlash across Canada over how it has dealt with a devastating hit that sent Montreal Canadiens forward Max Pacioretty to hospital with a cracked vertebra.

The NHL faced criticism on several fronts Thursday, with Montreal police launching a criminal investigation into the hit, Prime Minister Stephen Harper urging the league to change its safety policies and Air Canada threatening to withdraw sponsorship.

Canadiens chairman Geoff Molson also released an open letter to the team's fans expressing "frustration, disappointment and shock" over the hit, which was levelled by Bruins defenceman Zdeno Chara during a game on Tuesday, and the league's response.

Pacioretty was sent to hospital with a severe concussion and a fractured vertebra. He was released from hospital Thursday and returned home, according to a report in The Montreal Gazette.

Citing an anonymous source at Montreal General hospital, the injured Canadiens forward was able to leave his hospital bed and walk around for brief periods before his release.

Chara was given a penalty for interference and a game misconduct. On Wednesday, NHL senior vice-president Mike Murphy said he will face no further league discipline for the hit, which he called "a hockey play that resulted in an injury because of the player colliding with the stanchion and then the ice surface."

In the face of growing pressure, NHL Commissioner Gary Bettman defended the league's response Thursday, saying that while Pacioretty's injury is horrific, it's part of the game.

"Our hockey operations people are extraordinarily comfortable with the decision that they made," Bettman said in Washington. "It was a horrific injury, we're sorry that it happened in our fast-paced physical game, but I don't think whether or not supplemental discipline was imposed would change what happened."

Bettman also said the league can replace any sponsors that pull out.

"Air Canada is a great brand, as is the National Hockey League, and if they decide that they need to do other things with their sponsorship dollars, that's their prerogative."

The air carrier had sent a letter to Bettman earlier in the day, saying that it is "very concerned with the state of hockey today," and that the Pacioretty hit has caused it to reconsider its sponsorship of the NHL.

"From a corporate social responsibility standpoint, it is becoming increasingly difficult to associate our brand with sports events which could lead to serious and irresponsible accidents," the letter reads.

The company does not want to associate its brand "with sports events which could lead to serious and irresponsible accidents; action must be taken by the NHL before we are encountered with a fatality."

Unless Bettman takes immediate action and suspend the player involved, the letter said, Air Canada will withdraw its sponsorship of the league.

Political fallout

Montreal police confirmed Thursday that it has launched a criminal probe at the behest of Quebec's director of criminal and penal prosecutions, Louis Dionne.

"Like all police investigations, evidence will be gathered and an investigation report will be submitted (to the DCPP)," Dionne's spokesperson Martine Berube said Thursday. "(The DCPP) will then evaluate to see whether there's grounds for prosecution."

When asked what kind of punishment might result from the case, Berube said it was too soon to say.

Following an event in Toronto, Prime Minister Stephen Harper urged the NHL to deal with the growing number of serious injuries in the league.

"I want to express sympathy to Mr. Pacioretty, his family, his teammates," Harper told reporters, adding that he hopes to see the 22-year-old "have a full recovery and get back on the ice.

"I think we're all concerned, and I hope the league is concerned, about the number of very serious injuries we've seen in recent times -- and I do think that is something that they're going to have to address."

Gary Lunn, the federal minister responsible for sport, has said the league should take action following Chara's hit and put out a clear message that it was unacceptable.

Habs fans outraged

Police in Montreal have had to ask Canadiens fans to stop calling to file complaints against Chara. Police said their emergency lines started getting flooded shortly after the league announced there would be no suspension.

Pacioretty says he's "disgusted" that the NHL didn't suspend Chara for the hit.

"I am upset and disgusted that the league didn't think enough of (the hit) to suspend him," Pacioretty told TSN's Bob McKenzie Wednesday.

"I'm not mad for myself. I'm mad because if other players see a hit like that and think it's okay, they won't be suspended, then other players will get hurt like I got hurt."

"It's been an emotional day. I saw the video for the first time this morning. You see the hit, I've got a fractured vertebra, I'm in hospital and I thought the league would do something, a little something. I'm not talking a big number, I don't know, one game, two games, three games... whatever, but something to show that it's not right."

Dr. Blaine Hoshizaki, an expert in sport concussions at the Neurotrauma Impact Laboratory at the University of Ottawa, says while he doesn't know the details of the injury, there is a very real possibility that Pacioretty will never play for the NHL again.

"Not only the fractured vertebra, but also the severe concussion -- both can be career-ending injuries," he told CTV's Canada AM from Ottawa Thursday.

"Any time you damage the vertebrae, it's a serious injury. Fortunately, it's not displaced, which is very important because often these injuries can be resolved. However, it really depends on the type of fracture that occurred."

Hoshizaki notes that injuries such as this one are not altogether common, but nor is this an isolated case in hockey, since hits are an integral part of the game.

"It doesn't really matter if the hit was illegal or not illegal. The question that has to be asked is: is that risk that is part of the game reasonable for anyone who wants to play hockey to have to accept that sort of injury risk?"

With files from The Canadian Press