Police in Edmonton have carried out what is believed to be the largest fentanyl and W-18 bust in Canadian history.

Over a nine-month investigation dubbed Project Alchemy, a team of local and federal police officers seized 100 kilograms of the chemical compound used to make bootleg fentanyl — enough to produce 38 million fentanyl pills — as well as four kilograms of W-18 powder, and 3,200 fentanyl pills containing W-18 — another potent drug recently banned by Health Canada.

During a press conference on Thursday, Staff Sergeant Dave Knibbs said that two people, a man and a woman, were arrested this week and charged with 20 drug and firearm-related offenses.

Knibbs told VICE News that shipments of the fentanyl precursor — technically called N-phenethylpiperidinone (NPP), and used only in illicit fentanyl production — were discovered in four cardboard barrels by Canadian border guards at the Edmonton International Airport. The barrels, held together with bits of metal on the top and bottom, had been shipped from somewhere in China.

Knibbs said the NPP powder, which looks like brown sugar, was in plastic bags inside the barrels.

"They were hiding in plain sight," he said. "It wasn't concealed in anything else."

Fentanyl, a potent synthetic opioid 40 to 50 times stronger than heroin, has been linked to skyrocketing rates of overdoses in Canada and the US in recent years. According to the Centers for Disease Control and Prevention, overdose deaths in the US involving synthetic opioids, including fentanyl, increased by 80 percent from 2013 to 2014. While Canada does not track overdoses nationally, opioids such as fentanyl have been responsible for hundreds of deaths across the country over the last year.

Alberta has been ground zero for the problem, as nearly 300 people there died from fentanyl-related drug overdoses in 2015. In May, police in Calgary announced the province's first death related to W-18.

Early last June, police announced what was at the time the biggest fentanyl seizure in Edmonton, when they seized a drug shipment of 2,000 fentanyl pills.

The biggest exporter of illicit fentanyl and W-18 to Canada is said to be China. Last month, VICE News learned that Canadian border agents had seized three shipments of fentanyl from China, including a massive shipment destined for Toronto.

In April, Quebec City police seized $1.5 million worth of fentanyl powder and bills.