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Terror alert systems are standardised emergency population warning systems for describing and disseminating information about terrorism-related threats. They became more popular after the September 11 attacks on the United States in 2001.

National Counter-Terrorism Alert Level

• Certain - terrorist attack is imminent or has occurred
• Expected- terrorist attack is highly likely
• Probable - terrorist attack is likely
• Possible - terrorist attack could occur
• Low - terrorist attack is not expected

Administered by the Federal Attorney-General's Department.

Plan Vigipirate

• Yellow - to stress vigilance
• Orange - to warn of terrorist action
• Red - to warn of serious attempts
• Crimson - to warn of major attempts

International Terror Threat Level

• Low: An attack is deemed unlikely.
• Moderate: An attack is possible, but not likely.
• Substantial: An attack is a strong possibility.
• Severe: An attack is highly likely.
• Critical: An attack is imminent or has occurred.

Administered by the Garda Síochána.^[1]

Since November 2015, the international terrorism threat level is considered to be at moderate, meaning an attack is "possible but not likely".^[2][3]

• Blue - an unconfirmed information about real terroristic threat^[4]
• Yellow - a confirmed information about real terroristic threat
• Red - an information about committed terrorist act

• Low - an attack is unlikely
• Moderate - an attack is possible, but not likely
• Substantial - an attack is a strong possibility
• Severe - an attack is highly likely
• Critical - an attack is expected imminently or has occurred

Administered by the Joint Terrorist Analysis Centre

Homeland Security Advisory System

• Low - Low risk of terrorist attacks
• Guarded - General risk of terrorist attacks
• Elevated - Significant risk of terrorist attacks
• High - High risk of terrorist attacks
• Severe - Severe risk of terrorist attacks

Administered by the Department of Homeland Security

National Terrorism Advisory System

• Elevated Threat - Significant or high risk of terrorist attacks
• Imminent Threat - Critical risk of terrorist attacks