Social media websites like Facebook and Twitter - and others such as the Department of Homeland Security website - will be used by the Department going forward as part of a new National Terrorism Advisory System. It replaces the old color-coded Terror Alert System.

Officials say it will more effectively communicate information about terrorist threats by providing timely, detailed information to the public, government agencies, first responders, airports and other transportation hubs, and the private sector.

DHS says - in some cases, alerts will be sent directly to law enforcement or areas of the private sector. Other times, alerts will be issued more broadly to the American people through both official and media channels-including a designated DHS webpage. Alerts will include a clear statement that there is either an "imminent" or "elevated" threat.