Searchers dig through the rubble for victims of the inferno in Lac-Megantic, Quebec, on Monday while firefighters continue to hose down tanker cars to prevent more explosions. A runaway train derailed Saturday igniting tanker cars carrying crude oil, killing at least 13.

LAC-MEGANTIC, Quebec — Investigators searching for the cause of a fiery oil train derailment that wiped out a small town’s center and killed at least 13 people zeroed in on an earlier blaze on that same train, and the possibility that the series of actions that followed it might have somehow caused the locomotive’s brakes to fail several hours later.

Inspectors searched for remains in the derailment’s devastated epicenter after finally being cleared to enter the area late Monday — almost three days after the disaster. Nearly 40 people were still missing, not counting the 13 unidentified victims.

The rail tankers that blew up had a history of puncturing during accidents, but investigators acknowledged that it was too soon to tell whether that had been a factor in the explosions.

All but one of the train’s 73 cars were carrying oil. At least five of the train’s tankers exploded after coming loose early Saturday, speeding downhill nearly seven miles and derailing into the town of Lac-Megantic, near the Maine border.

Maude Verrault, a waitress at downtown’s Musi-Cafe, was outside smoking when she spotted the blazing train barreling toward 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,” Verrault said. She said she felt the heat scorch her back as she ran, but was too terrified to look back.

Transportation Safety Board investigator Donald Ross said Canada’s TSB has gone on record saying that it would like to see improvements on these 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 rail tanker 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,” 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 moving at 63 mph when it derailed.