In this Sept. 8, 2021, file photo, the Portsmouth Naval Shipyard is seen in Kittery. Credit: Robert F. Bukaty / AP

Portsmouth Naval Shipyard has delivered an attack submarine back to the U.S. Navy fleet a few days ahead of schedule.

The USS Annapolis underwent maintenance starting in June. About 400 employees of the Maine shipyard temporarily relocated to San Diego to do the work, the Navy said Wednesday.

The Navy said the sub was delivered back to the fleet six days ahead of schedule on Dec. 12. The sub will be homeported in San Diego.

The submarine is the fourth ship to be named for Annapolis, Maryland, which is home to the U.S. Naval Academy. It’s a Los Angeles-class nuclear-powered submarine. The sub was commissioned in 1992.