The Transportation Safety Board blamed a fire aboard a Westjet flight in June on spare lithium-ion batteries from an e-cigarette.

Flight 113 was on its way from Calgary to Vancouver on the morning of June 14 with 53 passengers and five crew on board, when a fire was reported in the aft cargo hold.

The crew declared an emergency, activated fire extinguishers, and returned to Calgary where the plane landed safely.

The TSB released its report into the incident Friday.

The board said a passenger inadvertently packed two spare batteries for his e-cigarette in his checked bag, a military-style backpack, while carrying his device into the cabin with him, as is required by regulations.

The board says a single damaged battery caused the fire, though it was not clear if the battery was damaged before getting to the airport or during baggage handling.

The TSB said the bag was screened before it was placed in the plane’s cargo hold, but security was looking for explosives, not dangerous cargo. The TSB said that while airline policies around e-cigarette batteries were clear, no one agency or person was responsible for enforcing them.

The danger around lithium-ion batteries has been well-reported, causing more than 200 fires on aircraft since 1991.