Prince George RCMP said several people broke into the building overnight.
There was evidence of copper pipe theft in the building, which caused damage to at least one natural gas line.
The explosion was ignited “due to the activities of one of the unauthorized persons within the building,” who were not aware of the the build-up of natural gas inside.
The investigation is ongoing, but police provided the update on Thursday because the explosion affected many people in Prince George, said Cpl. Jennifer Cooper.
“This investigation is incredibly complex and it will take time before investigators can make any further determinations,” she said.
Three people were taken to hospital after the explosion, including one who sustained serious injuries. Debris was scattered over several blocks.
Nine buildings had their windows blown out by the explosion, said the City of Prince George, with more than 50 businesses in the area damaged.
The abandoned building was the site of a shuttered Greek restaurant.