A U.S. Navy vessel that was being serviced inadvertently dumped hazardous material into the surrounding bay on Monday, the service said.

The Arleigh Burke-class Aegis guided missile destroyer USS Sampson was at the Vigor Shipyard on Harbor Island in Seattle, Washington, undergoing defueling operations when an estimated 2,000 gallons of diesel fuel spilled into the surrounding water.

“The Navy is working with shipyard contractors and spill response partners to resolve this quickly and responsibly,” the Navy said. “Active, 24-hour on-site recovery operations are underway and most of the fuel has been recovered.”

Initial estimates said the amount of oil spilled was closer to 5,000 gallons, but the number had decreased significantly.

The exact volume of the discharge is still being assessed, the Navy said.

The exact cause of the spill is under investigation.

Prior to the incident, the Navy installed an oil containment boon per Navy regulations, which helped to keep the spill contained.

Teams are utilizing three portable skimmers and a skimmer boat, along with oil-absorbent materials, to clean up a majority of the spill.

The containment boom and absorbent materials will remain until all recoverable oil is removed and the ship is moved to dry dock, the Washington State Department of Ecology said.

The U.S. Coast Guard determined that the U.S. Navy was at fault for the spill, according to the department.

Last August the Navy deployed the USS Sampson to the Caribbean in support of counter-narcotics operations.

The vessel returned to Naval Base San Diego, California, in September after being replaced in the U.S. Southern Command area of responsibility by the Arleigh Burke-class guided-missile destroyer USS Stockdale.