(CNN) -- Tanker cars involved in the calamitous derailment in Quebec in July were carrying fuel that was improperly described in shipping documents as being less volatile than it actually was, Canadian safety investigators said Wednesday.
The train was hauling highly volatile petroleum crude oil, a "Class 3, Packing Group II" product. But shipping papers identified the fuel as a less volatile "Packing Group III" product, investigator Donald Ross said.
The discovery helps explain "why the fuel ignited so quickly" after the derailment on July 6 in Lac-Megnatic, Ross said.
Forty-seven people were killed in the ensuing explosion and fire that leveled the tiny community's downtown.
But investigators did not say whether the shipping error was intentional or inadvertent, and left open the question about whether the error made a difference.
Both types of fuel -- Packing Group II and III -- are shipped in the same Class III tank cars, Ross said. Shipments of both classifications of fuel were allowed to travel through the Quebec town. And local firefighters said their emergency response would have been identical, he said.
But Ross said the investigation remains open, saying "Our work's not done."
Investigators say the events were set in motion when a fire was reported on the train.
The train's engine was shut down and the fire was extinguished, but a rail employee left the scene without setting enough brakes to keep the train from rolling. It then rolled down a grade into the center of Lac-Megantic.
Ross said it was the responsibility of the company receiving the shipment -- Irving Oil -- to properly identify the shipment.
Irving Oil, which was shipping the petroleum crude oil from the Bakken oil field in North Dakota to its refinery in St. John, New Brunswick, did not return a call to CNN for comment.
Ross also said the board's findings thus far hold significance beyond the Lac-Megnatic tragedy.
The incident "brings into question the adequacy of Class III tank cars" for transporting large quantities of low flash-point flammable liquids.
And, in letters to Canadian and U.S. regulators, the Transportation Safety Board of Canada (TSB) asked them to check the accuracy of train manifests.
"It's important that dangerous goods in transport be properly described. There are people that may have to handle that, that come in contact with that, and they need to know the hazards of what they're dealing with," he said.
The U.S. Pipeline and Hazardous Materials Safety Administration, said in a statement Wednesday that is issued a safety advisory in August addressing the issue of accurate labeling.
The U.S. Department of Transportation also launched an inspection operation to verify that shippers and rail carriers are properly classifying crude oil.