Two FBI reports, one of them fully declassified only this year, describe the likely, if limited, use of biological agents, in particular bubonic plague and anthrax, as part of a large-scale balloon, or “Fu-Go,” barrage launched by Imperial Japan on the United States and Canada in late 1944 and the first four months of 1945.

The bacteria, described as coming from balloons that landed in the Dakotas and Nebraska, were identified as anthrax. The pathogen was discovered in the gas (mostly hydrogen) that inflated the massive paper-laminated balloons.

The technology for the Fu-Go balloons was studied and helped to inform the United States own efforts to create the secretive E77 balloon bomb, which carried a biological agents payload.