Police have arrested a 61-year-old Montreal man for allegedly making threats against people involved in the shale gas industry.

An investigation began June 10 after a letter was sent to the French-language daily La Presse, threatening major consequences against the industry as of July 1.

The man was arrested early Thursday morning at his home in east Montreal. Provincial police spokesperson Daniel Thibaudeau said he was released and will appear in court to face terrorism-related charges.

"The conditions of his release are strict and they are put in place to ensure the person does not pose any further threat before he appears before the court," Thibaudeau said.

The letter sent to La Presse looked like a ransom note, with letters cut out of newspapers and pasted together.

Montreal police, RCMP and the Sûreté du Québec's anti-terrorism division are all involved in the investigation.

The letter mentioned the Liberal government's refusal to impose a moratorium on shale gas exploration. The letter warned, "We don't have the choice, we will impose our own moratorium and we'll take the necessary measures to have it respected."

The shale gas industry has become increasingly controversial in Quebec and other parts of Canada, including Nova Scotia and New Brunswick.

Shale gas is extracted from beneath the surface of the earth through a process called hydraulic fracturing, or "fracking." Opponents say the water-intensive process releases chemicals into the groundwater, threatening the environment and people's health. But proponents for the industry say it has the potential for major economic benefits.

recent protest in Montreal attracted thousands of people calling on the Quebec government to impose a 20-year moratorium on shale gas exploration.