( 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 .
Investigation finds tankers were carrying more volatile fuel than labeling showed . Development explains why fuel ignited so quickly in July derailment in Lac-Megnatic . Disaster claimed 47 lives as tankers exploded and leveled downtown area .