Arson was the cause of Sunday's fire and explosion at a southwest Omaha dry-cleaning store, the State Fire Marshal's Office said. Investigators found evidence of tampering with the natural gas system in the Wardrobe Spa near 168th and Harrison Streets, according to a statement released early Wednesday. The investigation determined that the cause of the explosion was not affiliated with the utility operator, Metropolitan Utilities District. No one was injured in the explosion and fire. The dry cleaner is closed on Sundays, and no one was inside the building. No nearby buildings were damaged.