F ive people were injured in an explosion Monday at an illegal marijuana grow house in Detroit, Michigan, according to authorities.

Four were transported to the hospital in critical condition, and one is stable, police said.

The people inside the building were allegedly making THC oil when the explosion happened, and investigators believe the THC was being extracted to use in vape cartridges when something in the process went wrong, according to local reports.

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"We believe at this time this was an illegal marijuana grow house and the explosion was related to THC," police said in a statement reported by NBC News. The investigation is ongoing, and no arrests have been reported.

Detroit Fire Chief James Harris said the incident occurred Monday night, with fire officials arriving at approximately 7:20 p.m.

"We arrived on scene there was heavy smoke, heavy fumes," Harris told Local 4. "The men and woman of the Detroit Fire Department did an awesome job rescuing people out of there and putting that fire out."

Frank Owens, a witness who heard the explosion, said he thought the noise was the weather at first because a storm passed through Detroit on Monday.

“We thought it was lightning and thunder because trees were down on the other lot,” Owens told the outlet. “They messed up a couple of cars. I heard a loud boom. I seen people run out half naked, with their clothes off.”

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A news crew with Local 4 said there were heavy fumes and they were told to stay away from the scene on Monday night. The scene was initially declared a "level one hazmat situation," according to Fox 2 Detroit.