By Alli Knothe Globe Correspondent

A vacuum cleaner was the cause of the $400 million fire in a nuclear-powered US Navy submarine in the Portsmouth Naval Shipyard last month that left seven people injured.

The vacuum cleaner, shipyard officials said Wednesday in a statement, was used to clean work sites at the end of shift. It was was stored in an unoccupied space.

They did not release specific information about how the vacuum cleaner started the fire, which took more than 12 hours for firefighters to extinguish.

Authorities have completed the first phase of the cleanup process, including taking the water out of the ship and installing temporary lighting and staging, the statement said.

The fire broke out May 23 aboard the USS Miami. The fire did not affect the Miami's nuclear propulsion spaces.

The statement said the ship’s nuclear propulsion plant was not operating at the time and it had been shut down for more than two months.

The “initial rough repair cost” for the sub is $400 milion, plus another 10 percent for secondary effects, including disruption to other planned Navy shipyard work, the statement said.