Mount Fuji and the city of Fujiyoshida are pictured on Dec. 12, 2021. (Aaron Kidd/Stars and Stripes)

A Japanese man birdwatching near Mount Fuji last week discovered an unexploded artillery shell, prompting an investigation by U.S. and Japanese authorities.

The unidentified man from Kanagawa prefecture found the object Sept. 24 and reported it to the Japan Ground Self-Defense Force’s Camp Kita-Fuji on Saturday, a spokesman with the Fujiyoshida police told Stars and Stripes by phone Monday. It wasn’t immediately clear why the man waited to report it.

Police responded to the area and confirmed the object is an unexploded shell, the spokesman said.

It measured about 10 inches long and 3 inches in diameter and appeared “quite old,” and may be a remnant of World War II, he said.

The shell was found about 330 yards from the nearest road; police were stationed near it over the weekend to prevent anyone from accidentally encountering it.

“Since it appears to have gunpowder inside, it could be dangerous,” the spokesman said.

It’s customary in Japan for some government officials to speak with the media on condition of anonymity.

An explosive ordnance team from Combined Arms Training Center Camp Fuji, a U.S. Marine Corps training base about 8 miles from where the shell was found, removed it Monday afternoon, according to a statement from Camp Fuji’s commander.

The team “determined it was a U.S. tank round” manufactured during World War II that was “likely fired by U.S. Forces in training” between the 1950s and 1960s, base commander Col. Neil J. Owens said by email Monday.

The shell was determined to be safe to move and “subsequently disposed” per standard procedures, Owens said.

A spokesman from Camp Kita-Fuji said the base always tracks ordnance used in training and is always aware if they explode or not, he said.