Investigators probing an oil train derailment and blaze which killed at least 13 people and wiped out the centre of a small town are focusing on an earlier fire on the same train and the possibility that the actions which followed it might have somehow caused the locomotive's brakes to fail several hours later.
Inspectors, meanwhile, are searching for remains in the wreckage after finally being cleared to enter the area - almost three days after the disaster. A total of 50 people are missing, including the 13 unidentified victims, and the death toll is expected to rise.
The rail tankers which exploded had a history of puncturing during accidents, but investigators acknowledged that it was too soon to tell whether that was a factor in the blasts.
All but one of the train's 73 cars were carrying oil. At least five of the tankers blew up after coming loose early on Saturday, speeding downhill for nearly 11 kilometers before derailing in the town of Lac-Megantic, near the Maine border.
Maude Verrault, a waitress at the 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," she said. She said she felt the heat scorch her back as she ran from the explosion, but was too terrified to look back.
The 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.
TSB investigator Donald Ross said Canada's TSB has gone on record as saying it would like to see improvements on those 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," Mr 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 travelling at 63mph (101kph) when it derailed.
Officials are also examining a locomotive blaze on the same train in a nearby town a few hours before the derailment. Mr Ross also said the locomotive's black box has been recovered, and investigators are examining whether the air brakes or the handbrake malfunctioned.
"The extent to which (the fire) played into the sequences of events is a focal point of our investigation," he said.
Saturday's blasts destroyed about 30 buildings, including a public library and the Musi-Cafe, a popular bar which was filled with revellers, 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 US - one that Canadian officials badly want - would be safer.
Raymond Lafontaine, whose son and two daughters-in-law are 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 remains part of a criminal probe and investigators are exploring all options, including the possibility that someone intentionally tampered with the train, said Quebec provincial police Sergeant Benoit Richard.
Canadian Transport Minister Denis Lebel said the train was inspected the day before the accident in Montreal and no deficiencies were found. He defended his government against criticism that it had cut back on rail safety measures. He said the railway remains a safe way to transport goods most of the time.
The train's owner, Montreal, Maine & Atlantic Railway, said it 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," he said.
Nantes Fire Chief Patrick Lambert said that when his crew intervened, the engine was shut off as 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, he 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''" Mr Lambert told reporters. "We were given our leave, and we left."
Edward Burkhardt, the president and chief executive of the railway's parent company Rail World, 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," he told the Canadian Broadcast Corp.
Meanwhile, crews were working to contain 27,000 gallons (100,000 litres) 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 that they could contain more than 85% 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 US Gulf Coast, has said rail 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 rail 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.