A national radiological dispersal device (RDD) exercise involving more than 24 federal, state, and local agencies was conducted in Philadelphia, Pennsylvania, the week of April 26, 2010, to test the Environmental Protection Agency’s response to a mock dirty-bomb attack. The exercise, called Liberty Radiation Exercise (Liberty RadEx), simulated a terrorist RDD attack in Philadelphia. The scenario envisioned a suicide bomber with explosives and cesium-137, a radioactive isotope.