Operation Dark Winter was the code name for a senior-level bio-terrorist attack simulation conducted from June 22–23, 2001. It was designed to carry out a mock version of a covert and widespread smallpox attack on the United States. Tara O'Toole and Thomas Inglesby of the Johns Hopkins Center for Civilian Biodefense Strategies (CCBS) / Center for Strategic and International Studies (CSIS), and Randy Larsen and Mark DeMier of Analytic Services were the principal designers, authors, and controllers of the Dark Winter project.