A 19-meter-long dock that washed up on the coast of Washington last month is tsunami debris from Aomori Prefecture, state authorities confirmed Thursday.

The dock was spotted by helicopter on a rugged beach within Olympic National Park on Dec. 18 after the crew of a fishing vessel reported it floating off the coast.

The Japanese Consulate in Seattle worked with Japanese government agencies to discover the dock's origins based on a serial number in a photo taken by a member of a cleanup team sent by the state.

The team removed more than 180 kg of organic matter from the dock to exterminate potential invasive species. State authorities are concerned that nonnative marine species from Japan could overrun local ecosystems.

State and federal agencies are working on a plan to remove the estimated 185-ton dock before its outer concrete shell is broken and its plastic foam core starts to break up and pollute the park.

It is unclear whether the dock is one of four ripped from the fishing port of Misawa, Aomori Prefecture, by the massive tsunami on March 11, 2011.

One of those docks was found washed up on an Oregon beach roughly 350 km south of the recent dock find last June.