It’s scary what people are putting into their bodies.

It was a carbon monoxide alarm that brought the Canadian authorities to the house in Liatris Drive, a quiet residential street lined with manicured gardens. As firefighters checked over the house to ensure its inhabitants were safe, something else caught their eye: kilograms of a mysterious powder sitting in the basement.

Soon afterwards, the police arrived at the house in Pickering, near Toronto, with a search warrant. They seized 33 identical handguns – and 53kg of the unidentified white and yellow powder.

Lab tests eventually revealed 42kg of the substance to be carfentanil – a drug the US Drug Enforcement Agency (DEA) has described as “crazy dangerous” and which authorities in the US have flagged as as potential chemical weapon. The local police force had unwittingly stumbled across what is believed to be the largest volume of the opioid ever seized in North America.

Developed in the 1970s as a tranquilizer for large animals such as elephants and bears, the synthetic opioid has also been studied as a potential chemical weapon by countries including the US, China and Israel. It is thought to have been deployed with disastrous effects when Russian special forces attempted to rescue hundreds of hostages from a Moscow theatre in 2002.

But it only burst into public view last year after officials across North America began to warn that it was being cut with heroin and other illicit drugs, leaving a rash of overdoses and deaths in its wake.