Investigators said Monday they hope to expand their search for 40 people missing two days after a runaway oil tanker train derailed and exploded, killing five and flattening part of a small Quebec town.
Police said the official death toll had not changed in Lac-Megantic since firefighters put out the raging inferno late Sunday, but admitted they had not yet been able to access parts of the charred wreckage.
"We couldn't search overnight," Quebec provincial police spokesman Benoit Richard said, explaining that much of the two-square-kilometer (0.77-square-mile) disaster area remained "extremely hazardous."
Police investigators hoped to conduct a more thorough search in the picturesque lakeside village once the smoldering debris cools and the fire marshal gives the green light, Richard told reporters.
The freight train operated by Montreal, Maine & Atlantic Railway derailed and exploded early Saturday, unleashing a wall of fire that tore through homes and businesses in Lac-Megantic -- population 6,000.
The fire leveled more than four blocks, including 30 buildings, and forced about 2,000 residents to flee their homes in the town, which is located 250 kilometers (155 miles) east of Montreal, near the US border.
Firefighters needed more than 18 hours just to contain the inferno, the cause of which was as yet unknown.
Survivors described a wall of flames as the runaway black tanker cars jumped the tracks, just as dozens of people were enjoying a summer night out in downtown bars and restaurants.
Montreal, Maine & Atlantic Railway said in a statement that the train had been transporting 72 carloads of crude oil when it derailed at around 1:20 am (0520 GMT) Saturday.
Rail company spokesman Christophe Journet told AFP the train had been stopped in the neighboring town of Nantes, around 13 kilometers (eight miles) west of Lac-Megantic, for a crew changeover.
For an as yet unknown reason, Journet said, the train "started to advance, to move down the slope leading to Lac-Megantic," even though the brakes were engaged.
There was no conductor on board when the train -- en route from the US state of North Dakota to a refinery in Canada's eastern New Brunswick province -- crashed, he said.
The company speculated that a temporary "shutdown" of the train in Nantes after a small engine fire "may have resulted in the release of air brakes on the locomotive that was holding the train in place."
But it added: "We don't have complete information concerning this incident, but will cooperate with government authorities as they continue their investigation" of the disaster.
Investigators from the Transportation Safety Board of Canada examined the locomotive on Sunday and the train's equivalent of a plane's "black box" recorder has been located.
Air quality tests showed no risk to the public, as locals waited in long lines Monday to retrieve pets, medications and a change of clothes from those homes still standing in the cordoned-off disaster zone.
Officials said they are mostly concerned about possible contaminants in the soil, but the derailment has also put the spotlight on the challenges of transporting oil by rail.
Construction of new pipelines -- hotly opposed by environmental activists -- has been unable to keep up with surging North American oil production, and so crude is largely being shipped by rail.
According to the Canadian Railway Association, oil shipments jumped from 500 container cars in 2009 to 140,000 so far this year.
Oil shipments helped turn around Montreal, Maine & Atlantic Railway, which the daily Globe and Mail said had been hurt by slumping demand for shipping forestry products over the past decade.
The head of the Canadian Railway Association, Paul Bourque, downplayed concerns, saying that millions of cars carrying freight are transported each year by train, virtually without incident.
After flying over the nearby Chaudiere River, Quebec Environment Minister Yves-François Blanchet told public broadcaster CBC that a "very thin" oil slick now stretches nearly 100 kilometers from Lac-Megantic.
He estimated that 100,000 liters of oil spilled into the river.
Officials said it was light crude oil that could mostly be recovered with minimal damage to the ecosystem.
But for now, residents have been told not to drink the water.