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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.

Wireless Emergency Alerts, operated by the Federal Communications Commission (FCC), allow mobile phone users to receive targeted messages about safety threats.

The National Terrorism Advisory System (NTAS) (2011-present)

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.

December 7, 2015 - DHS Secretary Jeh Johnson says his department will unveil a new national alert system to reflect a “new phase” of the terror threat.

December 16, 2015 - The first NTAS Bulletin is published. In it, the DHS describes the threat of “self-radicalized” individuals who may commit acts of terror inspired by extremist propaganda online. These bulletins are published every six months, describing current developments or general trends regarding threats of terrorism.

May 15, 2017 - The DHS renews an NTAS bulletin that warns of dangers posed by homegrown terrorists. The bulletin includes new warnings on the techniques used by terrorists, such as vehicle ramming attacks. The bulletin, which is set to expire in November, also includes new language describing foreign terrorist fighters.

- Warns of a credible terrorism threat against the United States.

- Warns of a credible, specific and impending terrorism threat against the United States.

Wireless Emergency Alerts (WEA)

2006 - Congress passes the Warning, Alert and Response Network (WARN) Act, requiring carriers that choose to participate to activate the technology by April 2012.

May 10, 2011 - FCC Chairman Julius Genachowski, FEMA Administrator Craig Fugate and New York City Mayor Michael Bloomberg announce a new alert system that will be available in New York City by the end of 2011 and eventually nationwide. It is originally called Personal Localized Alerting Network (PLAN) but its name later changes to WEA.
- 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.

- WEA alerts include warnings about extreme weather or other local emergencies, presidential alerts during national emergencies and AMBER alerts for missing children.

October 3, 2018 - The presidential alert system is tested for the first time, sending a message to nearly every phone in the country.

April 2, 2020 - In concert with the Federal Emergency Management Agency (FEMA), alerts are made available with geographically targeted information regarding the outbreak of the Coronavirus Pandemic.

Homeland Security Advisory System (HSAS) (2002-2011)

March 12, 2002-April 20, 2011 - The HSAS acts as a color-coded terrorism threat advisory scale. The level never goes below yellow.

August 10, 2006 - The DHS raises the threat level for commercial flights originating in the United Kingdom bound for the US to red. Raises level to orange for all commercial aviation in or destined for the United States. Three days later, the DHS lowers the threat level back to orange.

RED - Severe
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.

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.

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.