The Department of Homeland Security Advisory System was created by a Presidential Directive in order to provide a "comprehensive and effective means to disseminate information regarding the risk of terrorist acts to Federal, State, and local authorities and to the American people."
This system was unveiled by Tom Ridge, the Assistant to the President for the Homeland Security on March 12, 2002, in response to the September 11, 2001 Terrorist Attacks. It consists of 5 threat levels which are color coded. Risk includes both the probability of an attack occurring and its potential gravity. Low Condition (Green) is declared when there is a low risk of terrorist attacks. Guarded Condition (Blue) is declared when there is a general risk of terrorist attacks. Elevated Condition (Yellow) is declared when there is a significant risk of terrorist attacks. High Condition (Orange) is declared when there is a high risk of terrorist attacks. Severe Condition (Red) reflects a severe risk of terrorist attacks. The different levels trigger specific actions by federal agencies and state and local governments as well as the level of security at some airports.
The U.S. Attorney General is responsible for developing, implementing and managing the system and the decision whether to publicly announce Threat Conditions is made on a case-by-case basis by the Attorney General in consultation with the Assistant to the President for Homeland Security. Since January 2003, it has been administered in coordination with the United States Department of Homeland Security.
To date, the threat level has stood at yellow (elevated) for the majority of its time in existence. It has been raised to orange (high) five times: