Physical threats fall into two general categories. The first includes threats posed by explosives, such as the World Trade Center and Oklahoma City bombings. Also included are a number of less well-known attacks and thwarted attacks on facilities like electric power transformers and utility towers over the past decade. A much more significant aspect of this threat exists in the form of nuclear weapons, as well as chemical, biological and radiological threats.
The second category is electronic weapons designed to attack computer-based systems. Included here are radio-frequency devices that capture computer signals as they emanate from the equipment, and electromagnetic pulse and radio-frequency weapons that are intended to destabilize or destroy sensitive electronic components.