(CNN)Here is a look at terror alert systems in the United States. In 2011, the Department of Homeland Security (DHS) replaced the color-coded threat scale of the Homeland Security Advisory System with a new terror alert system, the National Terrorism Advisory System.
April 20, 2011 - The NTAS replaces the color-coded Homeland Security Advisory System (HSAS). When there is information about a threat, an alert will simultaneously be posted on the NTAS website and released to the news media for distribution. The advisories indicate whether the threat is "elevated," if there is no specific information about the timing or location, or "imminent," if the threat is impending.
- Warns of a credible terrorism threat against the United States.
- Warns of a credible, specific and impending terrorism threat against the United States.
2006 - Congress passes the Warning, Alert and Response Network (WARN) Act, requiring carriers that choose to participate to activate the technology by April 2012.
- WEA allows government officials to send emergency alerts to all subscribers with WEA-capable devices if their wireless carrier participates in the program. Consumers do not need to sign up for this free service.
- Consumers receive three types of alerts from WEA: alerts issued by the president, alerts involving imminent threats to safety and Amber Alerts. Subscribers can block all but presidential alerts.
- AT&T, Sprint, T-Mobile and Verizon are among participating carriers.
March 12, 2002-April 20, 2011 - The HSAS acts as a color-coded terrorism threat advisory scale. The level never goes below yellow.
Risk of terrorist attack: SEVERE
- Assign emergency response personnel and mobilize emergency response teams.
- Monitor, redirect or constrain transportation systems.
- Close public and government facilities.
- Increase or redirect personnel to address critical emergency needs.
Risk of terrorist attack: HIGH
- Coordinate necessary security efforts with law enforcement agencies and the National Guard.
- Take additional precautions at public events.
- Prepare to work at an alternate site or with a dispersed work force.
- Restrict threatened facility access to essential personnel only.
Risk of terrorist attack: SIGNIFICANT
- Increase surveillance of critical locations.
- Coordinate emergency plans with nearby jurisdictions.
- Assess further refinement of protective measures.
- Implement contingency and emergency response plans as appropriate.
Risk of terrorist attack: GENERAL
- Check communications with designated emergency response or command locations.
- Review and update emergency response procedures.
- Provide the public with necessary information.
Risk of terrorist attack: LOW
- Refine and exercise preplanned protective measures.
- Ensure that emergency personnel receive training on protective measures.
- Regularly assess facilities for vulnerabilities and take measures to address them.