The deadly derailment in Lac-Mégantic, Que., is fuelling a fierce political debate over rail safety and the implications of a sharp rise in shipments of petroleum products by train.

Prime Minister Stephen Harper, who likened the scene to a "war zone" during a weekend visit to the site, said there are many questions in the wake of the tragedy, but he cautioned Canadians not to leap to quick conclusions.

"It would really not be responsible to comment without knowing all the facts," he said Sunday. "The Transportation Safety Board of Canada is already undertaking a full investigation of this. I presume there’s also going to be a police investigation because of what I've heard has transpired here. So obviously, we will see the results of these investigations, and we will act on their recommendations."

It's still not clear what caused Saturday's derailment and explosion of the crude-carrying train owned by Montreal, Maine & Atlantic Railway Ltd., but it may have been caused by the release of the train’s brakes. At least 13 people were killed and about 50 are missing. Thirty downtown buildings were incinerated.

Families deserve answers

"I understand emotions here on this subject are very high," Harper said. "The information that I’ve seen about what has transpired — it may not be complete but it is very, very concerning about why this occurred. But that all said, there are proper authorities to investigate this, and I don’t want to say anything that would inhibit what the police or the Transportation Safety Board have to do in terms of arriving at their own conclusions and taking the steps they may ultimately deem necessary."

NDP Leader Tom Mulcair, who also visited the town on the weekend, suggested federal cuts and lax regulation were to blame.

"There has been a very large increase in traffic of petroleum products by rail — the regulation of that has to follow suit," he said.

While the immediate priority is to support the grieving families and town, the disaster has raised critical questions about rail safety, he said.

"My thoughts and prayers are with those families — that's our first priority today — but there's still lots of questions and those same families deserve answers to those questions," Mulcair, who represents the Quebec riding of Outremont, said Sunday.

'Culture of safety'

"One of the first roles of government is to ensure public safety, and we'll do everything we can to make sure the public is safe from this type of thing in the future. But people are also going to be asking a lot of questions how something like this could have happened."

Mike Winterburn, director of communications for Transportation Minister Denis Lebel, insisted the government is strengthening rail safety in Canada by investing $72 million in development of regulation, inspections and oversight activities, and research and development.

"Now is not the time to focus on partisan politics," he told CBC News. "Right now we need to focus on ensuring the safety and security of those in Lac-Mégantic. Rail safety oversight activities have not been cut. While increasing the number of front-line rail inspectors and auditors, Transport Canada has found administrative savings through the streamlining of information technology services and other efficiencies."

Winterburn also noted that the new Railway Safety Act that took effect this past May requires rail companies to create and maintain a "culture of safety" and includes tough penalties for violations. Since 2007, train accidents in Canada have decreased by 23 per cent, while train derailments have decreased by 26 per cent, he said.

Lebel held a news conference late in the day Monday in Lac-Mégantic and said the government has taken "concrete action to increase rail safety" and will continue to improve it.

"We cannot rest on our laurels," said Lebel. The minister said there have been no cuts to the number of federal rail inspectors and he noted that the train that derailed was actually inspected on July 5th and no deficiencies were found. He said there are more than 80 train inspectors working for the federal government.

If any regulations were broken in this incident, there will be consequences, he said. Lebel also said the opposition parties are spreading falsehoods about funding cuts.

NDP ethics critic Charlie Angus denied that Mulcair is playing politics with the tragedy, accusing the government of trying to change the channel. Mulcair was the first national leader on the ground after the disaster, he noted.

"They heard from the people there – the horror, the concern and the question that everyone in that community asks is why? How could this happen in our country? Certainly, that’s the question that Thomas Mulcair will be asking in the House of Commons and that’s I think what all Canadians want to know," he said. "That’s not politicizing – that’s the real, that's the honest thing that you see when you see a community destroyed like this."

NDP transport critic Olivia Chow said the disaster drives home the need for mandatory measures to improve rail safety in Canada, such as emergency braking systems and voice recorders.

"Unfortunately, Conservatives continue to rely on self-regulation," she said. "The federal government needs to act now, so families can trust that the trains they are riding and the trains riding through their communities, are as safe as they can be."

On CBC News Network's Power & Politics, Chow said the Conservatives have made cuts to railway safety but the NDP isn't necessarily linking them to the train derailment. She said her party is being up front about the cuts and recommendations that haven't been implemented but that it is "pointless to blame anybody."

Chow said everyone should work together to improve safety.

Liberal Leader Justin Trudeau cancelled a scheduled event at the Calgary Stampede and travelled to Lac-Mégantic Monday.

"I'm blown away obviously by the terrible destruction that we see behind us, that I got to visit but also by the strength and courage of the people who have come from across the country to help out, the sense of community that's drawn together, and just the strength of goodwill that is descending upon this devastated community," he told reporters.

Trudeau said he wouldn't comment on Mulcair's comments over the weekend. "There will be plenty of time for accusations, recriminations, mostly for investigations but right now we're still very much trying to offer these residents support," he said.