Canadian Prime Minister Stephen Harper compared the area to a war zone and said about 30 buildings were incinerated. Quebec provincial police Sgt. Benoît Richard said only a small part of the devastated area had been searched Sunday, more than a day since the accident, because firefighters were making sure all fires were out.
The train's 72 oil-filled tanker cars somehow came loose early Saturday morning, sped downhill nearly seven miles (11 kilometres) into the town, derailed and began exploding one by one. At least five exploded.
The eruptions sent residents of Lac-Megantic scrambling through the streets under the intense heat of towering fireballs and a red glow that illuminated the night sky. The district is a popular area packed with bars that often bustles on summer weekend nights. Police said the first explosion tore through the town shortly after 1 a.m. local time. Fire then spread to several homes.
Two tanker cars were burning Sunday morning, and authorities were still worried about them Sunday evening. Local Fire Chief Denis Lauzon said firefighters were staying 500 feet (150 metres) from the tankers, which were being doused with water and foam to keep them from overheating.
"This is an unbelievable disaster," said Harper, who toured the town Sunday. "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."
The growing number of trains carrying crude oil in Canada had raised concerns of a major derailment.
One death was confirmed Saturday. Police confirmed two people were found dead overnight and confirmed two more deaths Sunday afternoon. The charred remains were sent to Montreal for identification.
A coroner's spokeswoman said it may not be possible to recover some of the bodies because of the intensity of the blasts.
Witnesses said they feared for the lives of dozens who were at the nearby Musi-Cafe bar on a beautiful summer night in the town of 6,000, about 155 miles (250 kilometres) east of Montreal and just west of the Maine border.
David Vachon said no one had heard from a friend who had been celebrating a birthday there, or from the man's wife. "I knew a good portion of them, around 15 who are now missing ... It's terrible," he said.
Henri-Paul Audette headed to a shelter with hopes of reuniting with his missing brother, whose apartment was next to the railroad tracks, close to the place where the train derailed.
"I haven't heard from him since the accident," Audette said. "I had thought ... that I would see him."
About a third of the community was forced out of their homes.
Transportation Safety Board investigator Donald Ross said the black box of the locomotive has been recovered, but officials haven't been able to access much of the site.
Edward Burkhardt, the president and CEO of Rail World Inc., the parent company of Montreal, Maine & Atlantic Railway, said the train had been parked uphill of Lac-Megantic because the engineer had finished his run. The tanker cars somehow came loose.
"We've had a very good safety record for these 10 years," Burkhardt said. "Well, I think we've blown it here."
Joe McGonigle, Montreal, Maine & Atlantic's vice-president of marketing, said the company believes the brakes were the cause.
"Somehow those brakes were released, and that's what is going to be investigated," McGonigle said in a telephone interview Sunday. "We're pretty comfortable saying it is the brakes. The train was parked, it was tied up. The brakes were secured. Somehow it got loose."
Lauzon, the fire chief, said firefighters in a nearby community were called to a locomotive blaze on the same train a few hours before the derailment. Lauzon said he could not provide additional details about that fire since it was in another jurisdiction. McGonigle confirmed the fire department showed up after the first engineer tied up and went to a local hotel. Someone later reported a fire.
"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.
McGonigle said there was no reason to suspect any criminal or terror-related activity.
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 much of the 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 just 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.
"We think it is safe. We think we have a safe operation," McGonigle said of carrying oil by rail. "No matter what mode of transportation you are going to have incidents. That's been proven. This is an unfortunate incident."
Associated Press writer Rob Gillies and Charmaine Noronha contributed from Toronto.