Meanwhile, crews worked to contain oil spilling in the Claudiere River which feeds into the St. Lawrence.
The Quebec government issued a statement saying the leak was "serious, but under control." The statement said the oil could spread as far as Quebec City.
Floating barriers have been set up to try to stop the oil from heading downstream.
Quebec provincial police Sgt. Benoit Richard said Monday morning there was no searching overnight because the situation remained too dangerous.
He said only a small part of the devastated scene has been searched as firefighters made sure all flames were out.
Many of those missing were believed to have been drinking at a popular downtown bar when the explosions occurred and rescuers were still not able to reach the area, Richard said.
"Hopefully we'll be able to open up more areas for searching during the day," he said.
Firefighters on Monday were focusing their efforts on two oil-filled cars dousing them with water and foam in an attempt to keep them from overheating and exploding.
All but one of the train's 73 tanker cars were carrying oil when they somehow came loose early Saturday morning, sped downhill nearly seven miles into the town of Lac-Megantic, near the Maine border, and derailed, with at least five of the cars exploding.
About a third of the community of 6,000 was forced from of their homes by the explosion and flames.
The growing number of trains transporting crude oil in Canada and the United States had raised concerns of a major disaster, and this derailment was sure to bolster arguments that a proposed oil pipeline running from Canada across the U.S. would be safer.
"This is an unbelievable disaster," said Canadian Prime Minister Stephen Harper, who toured the town Sunday and compared it to a war zone. "This is an enormous area, 30 buildings just completely destroyed, for all intents and purposes incinerated. There isn't a family that is not affected by this."
Anne-Julie Huot, 27, said at least five friends and about 20 acquaintances remained unaccounted for.
"I have a friend who was smoking outside the bar when it happened, and she barely got away, so we can guess what happened to the people inside," Huot said. "It's like a nightmare."
A coroner's spokeswoman said it may not be possible to recover some of the bodies because of the intensity of the blasts. Spokeswoman Geneviève Guilbault said the bodies are so badly burned that identifying them could take a long time. She said none of the five bodies that have been found so far have been identified and two have been sent to Montreal for further analysis. All of the autopsies will be conducted in Montreal because there is no laboratory in town.
For the second day in a row, she urged families of the missing to come forward with details that could help them identify the bodies, such as tattoos, dental records, or objects that would contain the DNA of the deceased.
Health and civil security officials held a separate news conference and said some residents might be allowed back home later Monday.
The train's oil was being transported from North Dakota's Bakken oil region to a refinery in New Brunswick. 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.
Harper has called railroad transit "far more environmentally challenging" while trying to persuade the Obama administration to approve the controversial Keystone XL oil pipeline from Canada to the Gulf Coast. Greenpeace Canada said Sunday that federal safety regulations haven't kept up with the enormous growth in the shipment of oil by rail.
Officials with the Montreal, Maine & Atlantic Railway said that despite the disaster, they feel transporting oil by rail is safe.
"No matter what mode of transportation you are going to have incidents. That's been proven. This is an unfortunate incident," said Joe McGonigle, Montreal, Maine & Atlantic Railway's vice president of marketing.
He said the company believes the train's brakes were the cause. "The train was parked, it was tied up. The brakes were secured. Somehow it got loose," he said.
McGonigle said there was no reason to suspect any criminal or terror-related activity.
Wayde Schafer, a North Dakota spokesman for the Sierra Club, has predicted such a catastrophe ever since crude began leaving the North Dakota by rail in 2008.
"I think anybody could have foreseen this," said Schafer, whose downtown Bismarck office is just two blocks from a rail line that carries several mile-long oil trains daily through the heart of the North Dakota's capital city. "It seemed like a disaster waiting to happen and it happened."
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