Inspectors, meanwhile, searched for remains in the derailment's devastated epicenter after finally being cleared to enter the area late Monday — almost three days after the disaster. A total of 50 people were missing, including the 13 unidentified victims, and the death toll was sure to rise.
The rail tankers that blew up had a history of puncturing during accidents, but investigators acknowledged that it was too soon to tell whether that had been a factor in the explosions.
All but one of the train's 73 cars were carrying oil. At least five of the train's tankers exploded after coming loose early Saturday, speeding downhill nearly seven miles (11 kilometres) and derailing into the town of Lac-Megantic, near the Maine border.
Maude Verrault, a waitress at downtown's Musi-Cafe, was outside smoking when she spotted the blazing train barrelling toward her.
"I've never seen a train moving so fast in my life, and I saw flames ... Then someone screamed 'the train is going to derail!' and that's when I ran," Verrault said. She said she felt the heat scorch her back as she ran from the explosion, but was too terrified to look back.
The rail tankers involved in the derailment are known as DOT-111 and have a history of puncturing during accidents, the lead Transportation Safety Board investigator told The Associated Press in a telephone interview late Monday.
TSB investigator Donald Ross said Canada's TSB has gone on record saying that it would like to see improvements on these tankers, though he said it was too soon to know whether a different or modified tanker would have avoided last weekend's tragedy.
The DOT-111 is a staple of the American freight rail fleet. But its flaws have been noted as far back as a 1991 safety study. Among other things, its steel shell is too thin to resist puncturing in accidents, which almost guarantees the car will tear open in an accident, potentially spilling cargo that could catch fire, explode or contaminate the environment.
"It's too early to tell. There's a lot of factors involved," Ross said. "There's a lot of energy here. The train came down on a fairly significant grade for 6.8 miles before it came into the town and did all the destruction it did." He said the train was moving at 63 mph (101 kph) when it derailed.
Officials were also looking at a locomotive blaze on the same train in a nearby town a few hours before the derailment. Ross also said the locomotive's black box has been recovered, and investigators were examining whether the air brakes or the hand break malfunctioned.
"The extent to which (the fire) played into the sequences of events is a focal point of our investigation," Ross said.
The Saturday blasts destroyed about 30 buildings, including a public library and Musi-Cafe, a popular bar that was filled with revelers, and forced about a third of the town's 6,000 residents from their homes. Much of the area where the bar once stood was burned to the ground. Burned-out car frames dotted the landscape.
The derailment raised questions about the safety of Canada's growing practice of transporting oil by train, and was sure to bolster arguments that a proposed oil pipeline running from Canada across the U.S. — one that Canadian officials badly want — would be safer.
Raymond Lafontaine, whose son and two daughters-in-law were among the missing, said he was angry with what appeared to be a lack of safety regulations.
"We always wait until there's a big accident to change things," he said. "Well, today we've had a big accident, it's one of the biggest ever in Canada."
The area remained part of a criminal probe and investigators were exploring all options, including the possibility that someone intentionally tampered with the train, said Quebec provincial police Sgt. Benoit Richard.
Canadian Transport Minister Denis Lebel said the train was inspected the day before the accident in Montreal and no deficiencies were found. Lebel defended his government against criticism it had cut back on rail safety measures. He said the rails remain a safe way to transport goods the vast majority of the time.
The train's owners, Montreal, Maine & Atlantic Railway, said they believed brake failure was to blame.
Local fire chief Denis Lauzon said firefighters in the nearby town of Nantes, uphill from Lac-Megantic, were called to handle a locomotive blaze on the same train a few hours before the derailment.
Joe McGonigle, Montreal, Maine & Atlantic Railway's vice-president of marketing, said the fire was reported after the first engineer secured the train and went to a local hotel.
"We know that one of our employees from our engineering department showed up at the same time to assist the fire department. Exactly what they did is being investigated so the engineer wasn't the last man to touch that train, we know that, but we're not sure what happened," McGonigle said.
Nantes Fire Chief Patrick Lambert said that when his crew intervened, the engine was shut off per the standard operating procedure dictated by Montreal, Maine & Atlantic Railway. The blaze was extinguished within about 45 minutes. And that's where the fire department's involvement ended, Lambert said.
"The people from MMA told us, 'That's great — the train is secure, there's no more fire, there's nothing anymore, there's no more danger,'" Lambert told reporters. "We were given our leave, and we left."
Edward Burkhardt, the president and CEO of the railway's parent company Rail World, Inc., suggested that the decision to shut off the locomotive to put out the fire might have disabled the brakes. "An hour or so after the locomotive was shut down, the train rolled away," Burkhardt told the Canadian Broadcast Corp.
Meanwhile, crews were working to contain 100,000 litres (27,000 gallons) of light crude that spilled from the tankers and made its way into nearby waterways. There were fears it could flow into the St. Lawrence River all the way to Quebec City.
Quebec's Environment Ministry Spokesman Eric Cardinal said officials remained hopeful they could contain more than 85 per cent of the spill.
The growing number of trains transporting crude oil in Canada and the United States had raised concerns of a major disaster. Canadian Prime Minister Stephen Harper, who has been pushing the Obama administration to approve the controversial Keystone XL oil pipeline from Canada to the U.S. Gulf Coast, has said railroad transit is more "environmentally challenging" than pipelines.
The train's oil was being transported from North Dakota's Bakken oil region to a refinery in New Brunswick on Canada's East Coast. Because of limited pipeline capacity in the Bakken region and in Canada, oil producers are increasingly using railroads to transport oil to refineries.
The Canadian Railway Association recently estimated that as many as 140,000 carloads of crude oil will be shipped on Canada's tracks this year — up from 500 carloads in 2009. The Quebec disaster is the fourth freight train accident in Canada under investigation involving crude oil shipments since the beginning of the year.
Associated Press writers Gillies and Charmaine Noronha contributed from Toronto. James MacPherson contributed from Bismarck, North Dakota.