VAUGHAN, Ont. -- Ontario Provincial Police say 18 people have been charged in a cross-border investigation into illegal guns and drugs, including the deadly opioid fentanyl.

OPP say the investigation began by targeting multiple criminal groups trafficking firearms and drugs along the Highway 401 corridor between the Greater Toronto Area through to Montreal, and revealed drug trafficking to and from the U.S. as well.

The 18-month investigation -- known as Project Silkstone -- involved OPP, the RCMP, Quebec provincial police, Montreal police, the Canada Border Services Agency and the U.S. Drug Enforcement Agency.

Over the course of the probe, OPP say investigators seized 11,500 pills containing fentanyl, eight kilograms of cocaine and 7.5 kilograms of ecstasy, among other drugs and drug equipment.

Police also seized 23 firearms, including pistols and long guns, as well as Canadian and U.S. currency and $50,000 worth of casino chips.

OPP say the 18 people arrested in the investigation have been charged with a total of 81 offences under the Criminal Code and the Controlled Drug and Substances Act.