State officials are investigating whether the stern of a boat that washed up on Windward Oahu may be 2011 Japan tsunami debris.

The boat, about 20-foot long, was seen floating whole in Kahana Bay on Thursday.

By Friday afternoon, when it was reported, it had broken into pieces on rocks on the northward outer edge of the bay, according to the state Department of Land and Natural Resources.

The department said its staff retrieved pieces of the boat from the rocks and brought it ashore Saturday.

More pieces were recovered Tuesday — pieces containing Japanese words on the boat’s bow and Japanese registration numbers on the stern, department spokeswoman Deborah Ward said.

Ward said the department and the National Oceanic and Atmospheric Administration is working with the Consulate-General of Japan in Honolulu to verify if the boat is from tsunami debris.

If confirmed, it would be the fourth tsunami debris to have reached Hawaii shores and the 17th, for the U.S. and Canada.

Ward said two species of mussels on the boat, not currently known to be present in Hawaii, were collected by department staff and turned over to NOAA for possible identification.