Investigators probing an oil train derailment and blaze which killed at least 13 people and wiped out the centre of a small town are focusing on an earlier fire on the same train and the possibility that the actions which followed it might have somehow caused the locomotive's brakes to fail several hours later.
Inspectors, meanwhile, are searching for remains in the wreckage after finally being cleared to enter the area - almost three days after the disaster. A total of 50 people are missing, including the 13 unidentified victims, and the death toll is expected to rise.
The rail tankers which exploded had a history of puncturing during accidents, but investigators acknowledged that it was too soon to tell whether that was a factor in the blasts.
All but one of the train's 73 cars were carrying oil. At least five of the tankers blew up after coming loose early on Saturday, speeding downhill for nearly seven miles before derailing in the town of Lac-Megantic, near the Maine border.
Maude Verrault, a waitress at the Musi-Cafe, was outside smoking when she spotted the blazing train barrelling towards her.
"I've never seen a train moving so fast in my life, and I saw flames... Then someone screamed 'The train is going to derail!' and that's when I ran," she said. She said she felt the heat scorch her back as she ran from the explosion, but was too terrified to look back.
The tankers involved in the derailment are known as DOT-111 and have a history of puncturing during accidents, the lead Transportation Safety Board investigator told the Associated Press. TSB investigator Donald Ross said Canada's TSB has gone on record as saying it would like to see improvements on those tankers, though he said it was too soon to know whether a different or modified tanker would have avoided last weekend's tragedy.
The DOT-111 is a staple of the American freight rail fleet. But its flaws have been noted as far back as a 1991 safety study. Among other things, its steel shell is too thin to resist puncturing in accidents, which almost guarantees the car will tear open in an accident, potentially spilling cargo that could catch fire, explode or contaminate the environment.
"It's too early to tell. There's a lot of factors involved," Mr Ross said. "There's a lot of energy here. The train came down on a fairly significant grade for 6.8 miles before it came into the town and did all the destruction it did." He said the train was travelling at 63mph when it derailed.
Officials are also examining a locomotive blaze on the same train in a nearby town a few hours before the derailment. Mr Ross also said the locomotive's black box has been recovered, and investigators are examining whether the air brakes or the handbrake malfunctioned.