A fire that caused hundreds of millions of dollars in damage to a dry-docked submarine started when an industrial vacuum cleaner sucked up a heat source of some type that ignited debris inside the vacuum, the US Navy said Friday. The vacuum had no apparent defect and was not plugged in when the fire started aboard the USS Miami on May 23, the Portsmouth Naval Shipyard said in a statement. The Navy estimates the repair cost at $400 million. An investigation is ongoing, and additional details and a final damage assessment will be released later, the shipyard said. The Miami arrived at the shipyard in Kittery, Maine, in March for a 20-month overhaul. The fire damaged the torpedo room, crew quarters, and command and control areas in the front of the submarine, but the nuclear propulsion components at the back of the sub were not harmed. There were no torpedoes or other weapons on board at the time.