The National Terrorism Advisory System (NTAS) is a terrorism threat advisory scale used by the US Department of Homeland Security.
NTAS advisories may come in the following formats:
Bulletins - describes current developments or general trends regarding threats of terrorism;
Elevated Alerts - warns of a credible terrorism threat against the United States;
Imminent Alerts - warns of a credible, specific and impending terrorism threat against the United States.
The public is encouraged to stay informed and to listen to local law enforcement and public safety officials.