Canadian 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 told a press conference, explaining that much of the two-square-kilometer 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, he added.

The freight train operated by Montreal, Maine and 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 of the town's downtown area 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.

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 and 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 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 crashed, he said.

The company speculated that a "shutdown" of the train "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."