The production of hash oil has been determined to be the cause of an explosion that severely wounded three men in a South Side apartment on Wednesday, according to a Billings Police Department news release. 

BPD in partnership with the Billings Fire Department and Bureau of Alcohol, Tobacco, Firearms and Explosives determined it was a fuel-air explosion. 

The investigation is continuing.

The explosion, in the 100 block of South 29th Street, was reported at 8:30 a.m. Three men were taken to Billings hospitals, then transferred to burn centers in Denver and Salt Lake City. They are all in critical condition.

According to the Associated Press, methods abound for making hash oil, but one common practice is to force a solvent such as butane or propane gas or liquid through leafy cannabis, a process that separates its psychoactive material from buds, leaves and stems.

After the extraction, the hash-maker then releases the gas or boils off the liquid, leaving behind marijuana's psychoactive material in a concentrated form.

Without proper ventilation, though, the gases can pool in a room, where a spark from an appliance can trigger a severe explosion, knocking buildings off their foundation in some cases.

Colorado's Legislature is considering a law to make it a felony for amateurs to make hash oil, in response to explosions and injuries resulting from amateur hash oil production. At least 30 people were injured last year in 32 butane explosions involving hash oil in Colorado, according to the Rocky Mountain High Intensity Drug Trafficking Area, a state-federal enforcement program.

In Montana, two explosions last year, in Hamilton and at the University of Montana, were blamed on hash oil manufacturing.

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Managing editor at The Billings Gazette.