Investigators believe the crude oil aboard a Canadian train that derailed in the center of a small Quebec town in July, killing 47 people, was “violently” explosive and likely not correctly labelled, the Globe and Mail newspaper reported on Wednesday.

The Globe’s report, quoting unidentified sources who have seen correspondence between the Canadian Transportation Safety Board and U.S. regulators, said the TSB was worried about how oil from the Bakken region of North Dakota is labelled, and about whether tanker cars are up to the task of transporting them.

The train, hauling 72 tanker cars of Bakken crude, was parked uphill of the Quebec town of Lac-Megantic when it rolled away, accelerated on a downhill grade and derailed and exploded in vast fireballs in the center of town.

The Transportation Safety Board scheduled a news briefing for 3.30 British Time.

The fireballs had already raised concern about the cargo, given that crude oil does not normally explode very readily. Bakken crude is lighter, and hence more volatile, than crude from some other areas.

The Globe said the cargo was labelled as a “group 3 product”, the normal classification category for crude oil. But more explosive oil should be labelled group 2, a label that affects how emergency services deal with an accident.

“Safety board investigators argue the shipment did not conform with strict federal classification standards,” the Globe said. “Instead, officials concluded the lighter, more volatile crude should have been classified as a more explosive group 2 oil.”

The board was to announce new recommendations for the rail industry at its briefing. It has already said trains carrying dangerous goods must not be left unattended on a main track, and two “qualified persons” must run any train that hauls dangerous goods.

The train in the Lac-Megantic crash, operated by now-bankrupt railroad Montreal Maine & Atlantic, had a single engineer aboard when it was parked for the night on the main line.

Changes in crude labelling could have implications for the rising volumes of crude-by-rail shipments across North America, which has gained in popularity as pipelines fill to capacity.