At a rural property an hour outside Vancouver in October, Canadian police found 2.5 million doses of fentanyl and 528 gallons of chemicals in a shipping container and a storage unit. Six months earlier, they raided a home in a cookie-cutter Vancouver subdivision packed with barrels of fentanyl-making chemicals, glassware and lab equipment.

Thousands of miles away outside Toronto, police in August found what is believed to be the largest fentanyl lab so far in Canada — hidden at a property 30 miles from the U.S. border crossing at Niagara Falls, N.Y.

U.S. authorities say they have little indication that Canadian-made fentanyl is being smuggled south in significant quantities. But at a time when record numbers of people are dying from overdoses in the United States, the spread of clandestine fentanyl labs in Canada has the potential to undermine U.S. enforcement efforts and worsen the opioid epidemic in both nations.

Investigators in Canada say the labs are producing fentanyl for domestic users and for export to Australia, New Zealand and, they assume, the United States.