LAC-MEGANTIC, Canada (AFP) - The head of the US rail company at the center of Canada's worst train disaster in recent history on Tuesday blamed firefighters for the deadly derailment, as investigators combed through smoldering debris for evidence.

Residents of the small Quebec town of Lac-Megantic, part of which was flattened by the blast and subsequent inferno on Saturday that killed at least 13 people, began returning to their homes.

About three dozen people are still missing following the disaster, which unleashed a wall of fire that tore through homes and businesses in Lac-Megantic, located east of Montreal near the US border.

The chairman of the Montreal, Maine & Atlantic Railway, Edward Burkhardt, accused firefighters of releasing the train's brakes when it was stopped in Nantes, around 13km west of Lac-Megantic, for a crew changeover.

Those firefighters had been called to douse a small fire in one of the train's five locomotives.

Burkhardt told the daily La Presse that Nantes firefighters "showed up and put out the fire with a fire extinguisher. To do that they also shut down the first locomotive's engines. This is what led to the disaster."

He explained that the train's brakes were powered by the locomotive and would have disengaged when it was shut down, causing the driverless train to start rolling downhill towards Lac-Megantic.

By the time the company was informed of the shutdown, the train - en route from the US state of North Dakota to a refinery in Canada's eastern New Brunswick province - had already reached the town, he said.

MMA trains will no longer be left unattended, he vowed, noting that the company had launched an internal investigation.

Nantes Fire Chief Patrick Lambert however dismissed Mr Burkhardt's accusations, saying the 12 firefighters who responded to the locomotive engine fire followed all of the proper procedures.

The fire leveled more than four blocks, including 30 buildings, and forced about 2,000 of the town's 6,000 residents to flee. Many of those people began returning home on Tuesday.

"I know there is a lot of anger" in the small community, Mr Burkhardt told Canada's public broadcaster CBC.

The MMA chief had been expected in Lac-Megantic later Tuesday, but now said he would visit in the coming days, adding: "I hope I'm not going to get shot." Furious residents told AFP that a visit from Mr Burkhardt was already too late.

"Is he scared?" one 53-year-old man asked.

He must "at least apologise to us - that would calm things down a bit," said another resident.

Mr Burkhardt told CBC: "We want to cooperate with the town and with the residents in helping them getting back on their feet, accepting claims for their loss and assuring that something like this never happens again." "We're going to try to make people whole as best as we can," he said.

Investigators from Canada's Transportation Safety Board said they will focus on the train's brakes, as well as MMA's policies for securing stopped trains.

They will also look at possible inadequacies of the tanker cars for transporting flammable materials, and what caused the original locomotive fire in Nantes.

"It's very important to know exactly who did what - who was there, what did they do," TSB lead investigator Donald Ross told reporters, adding it was too soon to assign blame.

He did say the train began rolling moments after firefighters and a company official left the train unattended on the tracks in Nantes, after extinguishing the locomotive fire.

"The train started to roll without anybody on board," Mr Ross said.

It then picked up speed on the slope and derailed when it hit a curve in Lac-Megantic, and the rail cars piled up.

Fourteen TSB inspectors were on site Tuesday, "identifying and taking selective components for further analysis to our engineering lab in Ottawa," Mr Ross said.

He also noted that the tanker cars used by MMA to transport crude lacked shock-resistant "mud shields", but said they could be used to transport oil.

A small area of downtown Lac-Megantic remained closed off Tuesday as the clean-up began, with officials fearing that mopping up machinery could spark a fire in the sewers.

Quebec provincial police Sergeant Benoit Richard said several officers had to be taken out of the secure red zone and treated for unspecified health problems.

A precautionary boil water order remains in place while the town's water treatment plant is offline, but officials said the water was safe to drink.