(CNN)Here is a look at terror alert systems in the United States. In 2011, the US Department of Homeland Security (DHS) replaced the color-coded threat scale of the Homeland Security Advisory System (HSAS) with a new terror alert system, the National Terrorism Advisory System (NTAS). Also, Wireless Emergency Alerts (WEA), operated by the Federal Communications Commission (FCC), allow mobile phone users to receive targeted text messages about safety threats.
April 26, 2011 - The National Terrorism Advisory System (NTAS) replaces the color-coded Homeland Security Advisory System (HSAS).
- Describes current developments or general trends regarding threats of terrorism.
- 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 25, 2011 - The Homeland Security Advisory System 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 pre-position 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 armed forces or law enforcement agencies.
- Take additional precautions at public events.
- Prepare to work at an alternate site or with a dispersed work force.
- Restrict 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.