The increased interest in possible terrorist attacks using chemical and biological weapons has brought to light a, literally, lost chapter in American research in this area. After World War II, the US discovered the extensive biological and chemical work done by the Japanese Unit 731 in China. Many members of this unit had escaped China at the end of the war and were later apprehended in Japan. In return for immunity from prosecution, the Unit 731 members provided extensive information on their wartime activities. This led to further research by the US Army in the 1950s and 60s. But after a number of sheep were accidentally killed by a nerve gas test in 1969, this research came to the public's attention. Given the anti-military attitudes of the time (the Vietnam war), America's biological and chemical weapons research came under furious attack. The researchers, and the bureaucrats that ran the program, realized that a vigorous investigation of their work would reveal the secret tests on the American population in the 1950s, not to mention the relationship with Unit 731. So tons of documents were destroyed in an attempt to avoid embarrassing revelations. Unfortunately, lots of useful stuff went up in smoke as well. But no one knew that then, and efforts are (quietly) being made to reassemble some of the lost material. The lost test results covered things like the effectiveness of chemical and biological weapons when used on different types of targets, different weather conditions and the like. Some results were published, or stored in places that were forgotten. What cannot be retrieved will have to be redone, at great expense.