The root causes of the 1989 Montreal massacre led to a heated question period exchange between NDP Leader Tom Mulcair and Justice Minister Peter MacKay Tuesday after MacKay said Canadians may never understand what occurred.

Liberal MP Marc Garneau asked MacKay a question about the government's new firearms bill, which would reclassify some firearms, including the Swiss Arms rifle. Garneau said the survivors of the shooting at École Polytechnique oppose that bill, C-42.

MacKay said Canada has to work continually to support victims and hold offenders accountable.

"This week, we remember the horrific events that took place in Montreal at École Polytechnique 25 years ago, and while we may never understand what occurred — why this happened, why these women were singled out for this horrific act of violence, we have to stand together," he said.

Mulcair seized on that comment later in question period and asked whether MacKay wanted to correct his comments.

"We know why these women were singled out. It's because they were women. That's what Marc Lepine wrote in a manifesto," Mulcair said.

MacKay struggled to complete his thoughts as he responded to Mulcair.

"Of course they were singled out because they were women," MacKay said.

"To try to make this a partisan issue on such a day, Mr. Speaker, is deeply disappointing."

Outside question period, MacKay told reporters he was referring to not understanding the insanity and the level of violence.

Speaking to reporters outside the House of Commons, Liberal Leader Justin Trudeau said MacKay needs to spend more time listening to survivors of the shooting. He said every year as the anniversary approaches, it becomes clear the government doesn't understand what happened or the importance of remembering the event.