The Mayerthorpe Incident occurred on March 3, 2005 on the property of James Roszko in Rochfort Bridge, northwest of Edmonton near the town of Mayerthorpe, in the Canadian province of Alberta. With a Heckler & Koch 91, a civilian version of a military assault rifle, Roszko shot and killed Royal Canadian Mounted Police Constables Peter Schiemann, Anthony Gordon, Lionide Johnston, and Brock Myrol as the officers were executing a property seizure on the farm. This was the largest casualty toll for the RCMP in a single operation since 1962 when three constables were killed at Kamloops, B.C.

The incident
A memorial service for the slain officers was held in Edmonton on March 10, 2005 and televised nationally on CBC. Prime Minister Paul Martin and Governor General Adrienne Clarkson both spoke at the service. Many police officers from Canada and the United States were in attendance.
On May 19, 2005, Queen Elizabeth II, attended a ceremony in honour of the slain officers at the RCMP Training Academy in Regina, Saskatchewan.
The CBC program the fifth estate also made a documentary about the incident, which first aired on December 7, 2005. The full documentary can be downloaded from the CBC website (see links below).
A by-product of the massacre was the halt to marijuana decriminalization legislation in the Canadian Parliament. Only days before the massacre, the leader of the Marijuana Party endorsed the Liberal Party after the government introduced a bill to decriminalize use and possession of marijuana. In the wake of the tragedy, the Liberals shelved the bill, and it was predicted that no action would take place for at least a year. Before a year had elapsed, Martin's government was defeated and the Conservative Party, which opposed decriminalization, was elected. Many media commentators suggested the massacre was an argument against marijuana decriminalization; some others argued the incident only occurred because the officers were on a drug raid. As the details of Roszko's violent history and hatred for the police became apparent, these arguments generally faded from the public discourse.
The Fallen Four Memorial Society was founded to honour the slain policemen. The group intends to create the Fallen Four Memorial Park in Mayerthorpe, but its opening is not expected until at least mid-2008 due to limited amounts of funding and donations as of February 2007.

Charges are laid in 2007