In one of the longest-held secrets of the Cold War, the U.S. Army explored the potential for using radioactive poisons to assassinate important individuals" such as military or civilian leaders, according to newly declassified documents. Approved at the highest levels of the Army in 1948, the effort was a well-hidden part of the military's pursuit of a "new concept of warfare" using radioactive materials from atomic bombmaking to contaminate swathes of enemy land or to target military bases, factories or troop formations. Military historians who have researched the broader radiological warfare program said in interviews that they had never before seen evidence that it included pursuit of an assassination weapon. No targeted individuals are mentioned in references to the assassination weapon in the government documents declassified in response to a Freedom of Information Act request filed by the AP in 1995. The decades-old records were released recently to the AP, heavily censored by the government to remove specifics about radiological warfare agents and other details. The documents give no indication whether a radiological weapon for targeting high-ranking individuals was ever used or even developed by the United States. They leave unclear how far the Army project went. One memo from December 1948 outlined the project and another memo that month indicated it was under way. The main sections of several subsequent progress reports in 1949 were removed by censors before release to the AP. The broader effort on offensive uses of radiological warfare apparently died by about 1954, at least in part because of the Defense Department's conviction that nuclear weapons were a better bet. Whether the work migrated to another agency such as the CIA is unclear.