Five people were hurt in an explosion at a Detroit building that authorities said was an illegal marijuana grow house, and officials suspect the blast is related to THC vape cartridges.

Detroit police said fire crews arrived at the scene before 5:30 p.m. Monday.

"We believe at this time this was an illegal marijuana grow house and the explosion was related to THC," police said in a statement.

NBC affiliate WDIV of Detroit reported that investigators believe the THC was being extracted to use in vape cartridges.

Police said four people were hospitalized in critical condition and one as stable.

Neighbors who heard the explosion were startled.

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

WDIV crews were held back from the scene because of strong fumes.

Elisha Fieldstadt is a breaking news reporter for NBC News.