Investigators from the Seattle Police Department's arson-bomb squad determined the explosion in a vacant apartment unit Tuesday was caused by butane tanks stowed in a freezer, KOMO-TV, Seattle, reported.
Seattle police detective Renee Witt said butane is used to extract THC from marijuana plants.
CNN reported the blast's impact knocked one of the building's walls 6 inches off its foundation but did not cause a fire.
It wasn't known whether the site was a legal operation, KOMO said.
No injuries were reported.
Firefighters evacuated a flower shop in the front of the building. The building's utilities were shut off and the building was being inspected for structural integrity, police said.
Narcotics detectives were notified by emergency crews of marijuana in the freezer and a bedroom, and a growing operation in the basement, CNN reported.
Voters in Washington and Colorado last year approved the regulated sale of recreational marijuana to adults. While Colorado's law was effective on New Year's Day, Washington won't open marijuana outlets until later this year.