A brake failure may have led to a runaway freight train laden with crude oil derailing, exploding and destroying the centre of a Canadian town, according to the train's operator.
At least five people are dead and 40 others missing after the driverless train rolled downhill and into the centre of Lac-Megantic, Quebec, before exploding on Saturday.
The death toll is expected to rise as rescuers scour the town centre; the crash saw flames shoot into the sky and burn through the night.
The train's operator, Montreal, Maine & Atlantic, said the train locomotive's brakes somehow became disengaged as it was parked at a siding on a hill outside of the town. It did not know how that had happened.
"One fact that has emerged is the locomotive ... was shut down subsequent to the departure of the engineer ... which may have resulted in the release of air brakes on the locomotive that was holding the train in place," a company statement said. It was investigating other possible reasons, it added.
The crash happened shortly after 0500 GMT when the train derailed in the lakeside town of about 6,000 people, near the border with the US state of Maine. At least five of the train's 72 oil-filled cars exploded.
At least 30 buildings are believed to have been destroyed in the blast, and up to 1,000 people have been moved from the area. An aerial photograph showed widespread devastation in the town centre.
Al Jazeera's Daniel Lak, reporting from Lac-Megantic, said the explosion had wrecked an area that included popular restaurants and cafes.
After visiting the scene Stephen Harper, the Canadian prime minister: "This is an enormous area, 30 buildings just completely destroyed, for all intents and purposes incinerated,'' Harper said. "There isn't a family that is not affected by this. This is a very big disaster."
A witness told a local broadcaster the town centre had been crowded at the time of the derailment. Radio-Canada reported that one building at the centre of the town was a bar popular with young people.
"Many parents are worried because they haven't been able to communicate with a member of their family or an acquaintance," Mayor Colette Roy-Laroche told the local channel.
The Canadian Broadcasting Corporation said four pressurised tank carriages blew up after the train, which had 73 carriages in all, came off the rails. Residents told reporters they had heard five or six large blasts.
Fire officials said they feared more of the tanker carriages were at risk of exploding. About 30 buildings in the town centre were destroyed, some by the initial blast and others by the subsequent fire, they said.
Lac-Megantic is part of Quebec's Eastern Townships region, an area popular with tourists that is close to the order with Maine and Vermont. Quebec is a predominantly French-speaking province in the eastern half of Canada.
Our correspondent said that some residents and police had said that the oil may be spilling into the aqueducts and the river.