(CNN) -- Here is some background information about terror alert systems in the United States. In 2011, the U.S. Department of Homeland Security 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, the Personal Localized Alert Network (PLAN), operated by the Federal Communications Commission (FCC), allows mobile phone users to receive targeted text messages about safety threats.
The National Terrorism Advisory System (NTAS) (2011-present): April 26, 2011 - The National Terrorism Advisory System (NTAS) replaces the color-coded Homeland Security Advisory System (HSAS).
When there is information about a threat, an NTAS Alert will simultaneously be posted on the National Terrorism Advisory System's web site and released to the news media for distribution. The advisory will indicate whether the threat is Elevated, if there is no specific information about the timing or location, or Imminent, if the threat is impending or very soon. NTAS Alerts carry an expiration date and will be automatically canceled on that date. If the threat information changes, the Secretary of Homeland Security may announce an updated NTAS Alert.
Personal Localized Alerting Network (PLAN): 2006 - Congress passes the Warning, Alert and Response Network (WARN) Act, requiring carriers that choose to participate to activate PLAN technology by a deadline determined by the FCC, which is April 2012.
May 10, 2011 - FCC Chairman Julius Genashowski, FEMA Administrator Craig Fugate, and Mayor Michael Bloomberg unveil the Personal Localized Alerting Network (PLAN), a new national alert system that will be available in New York City by the end of 2011 and eventually nationwide. - PLAN allows government officials to send emergency alerts to all subscribers with PLAN-capable devices if their wireless carrier participates in the program. Consumers do not need to sign up for this free service. - Consumers will receive three types of alerts from PLAN: alerts issued by the president, alerts involving imminent threats to safety, and Amber Alerts. Subscribers will be able to block all but presidential alerts. - AT&T, Sprint, T-Mobile, and Verizon are among participating carriers. - All cell phones were required to have a chip that receives alerts by April 7, 2012.
Homeland Security Advisory System (HSAS) (2002-2011): 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.
September 10, 2002 - Raised from yellow to orange.
September 24, 2002 - Lowered from orange to yellow.
February 7, 2003 - Raised from yellow to orange.
February 27, 2003 - Lowered from orange to yellow.
March 17, 2003 - Raised from yellow to orange.
April 16, 2003 - Lowered from orange to yellow.
May 20, 2003 - Raised from yellow to orange.
May 30, 2003 - Lowered from orange to yellow.
December 21, 2003 - Raised to orange from yellow.
January 9, 2004 - Lowered from orange to yellow.
August 1, 2004 - Raised to orange from yellow for targeted areas in Washington DC, New Jersey and New York.
November 10, 2004 - Lowered back to yellow from orange for the targeted areas.
July 7, 2005 - Raised to orange for the mass rail and transit system after subway attacks in London.
August 14, 2005 - Lowered back to yellow.
August 10, 2006 - The U.S. Department of Homeland Security raises the threat level for commercial flights originating in the United Kingdom bound for the U.S. to red, the highest level ever. Raises level to orange for all commercial aviation in or destined for the U.S.
August 13, 2006 - Homeland Security lowers the threat level for commercial flights from the UK back to orange. Remains at orange for all domestic and international flights.
April 26, 2011 - The National Terrorism Advisory System (NTAS) replaces the color-coded Homeland Security Advisory System (HSAS).
RED - "Severe" 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.
ORANGE - "High" 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.
YELLOW - "Elevated" 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.
BLUE - "Guarded" 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.
GREEN - "Low" 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.