OTTAWA — It was an inside job of sorts. Thieves with access to a warehouse and a careful plan loaded up trucks and, over time, made off with $18 million of a valuable commodity.

The question is what was more unusual: that the commodity in question was maple syrup, or that it came from something called the global strategic maple syrup reserve, run by what amounts to a Canadian cartel.

On Tuesday, the police in Quebec arrested three men in connection with the theft from the warehouse, which is southwest of Quebec City. The authorities are searching for five others suspected of being involved, and law enforcement agencies in other parts of Canada and the United States are trying to recover some of the stolen syrup.

Both the size and the international scope of the theft underscore Quebec’s outsize position in the maple syrup industry.