From the record
“Mitchell Recreation Area is a small picnic area located in the Fremont-Winema National Forests, Lake County, Oregon, near the unincorporated community of Bly. In it stands the Mitchell Monument, erected in 1950, which marks the only location in the United States where Americans were killed during World War II as a direct result of a Japanese balloon bomb.”
Wikipedia article Mitchell Recreation Area, CC BY-SA 4.0.
One of 14 National Register listings published here for Lake County · listed in 2003 · category: Site · built in 1950.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service