Canadian investigators say a runaway train that blew up in July, killing 47 people, was hauling a more flammable, gasoline-like fuel than the crude oil it was supposed to be carrying.
The revelation by Canada's Transportation Safety Board (TSB) has raised questions about how accurately dangerous goods are identified on North American railroads.
The tanker train derailed and exploded in the middle of the small Quebec town of Lac-Megantic. It was North America's deadliest rail disaster in two decades.
In an update on its probe into the 6 July disaster, the TSB urged US and Canadian regulators to better document the transport of dangerous goods, and said it is also looking at the safety of the rail cars that are used to move oil. The TSB can recommend changes but not impose them.
The tanker train caused hundreds of millions of dollars in damages as well as the 47 deaths in what was the continent's deadliest rail disaster in two decades.
The calamity focused attention on rising volumes of crude-by-rail shipments across North America, which have gained in popularity as pipelines fill to capacity.
The fuel being shipped through Lac-Megantic by the now-bankrupt Montreal Maine & Atlantic Railway came from a US portion of the Bakken oilfield and was headed for an Atlantic Canadian refinery. The TSB said documents showed the cargo belonged to the Packing Group III category, which describes regular crude oil.
But tests showed the oil actually belonged to the Packing Group II category, which has the characteristics of gasoline and is much more explosive.
"The lower flashpoint of the crude oil explains in part why it ignited so quickly," TSB chief investigator Donald Ross told a news conference.
The TSB made clear the cargo would have been handled the same way, regardless of fuel classification, under current rules. Its investigation will now shift to the cars used on the train: old-style DOT111 tanker cars, which lack twin hulls or extra strengthening.