The HSAS threat level has changed 17 times as of September 2009. In August 2004, DHS began identifying specific sectors under possible threat, including aviation, financial services, and mass transit.
The threat level has been raised to Severe only once, which applied only to flights coming from the United Kingdom:
* August 10–14, 2006, in response to British law enforcement announcing it had disrupted a major terror plot to blow up an aircraft, DHS raised the threat level for commercial flights from the United Kingdom to the United States to Severe.
On a nationwide level, it has been raised to High five times:
* September 10–24, 2002, the first anniversary of the September 11, 2001 attacks.
* February 7–27, 2003, near the end of the Muslim religious holiday Hajj. Intelligence reports suggested the possibility of terrorist attacks against "apartment buildings, hotels, and other soft or lightly secured targets."
* March 17 – April 16, 2003, around the beginning of U.S. and Coalition military action in Iraq.
* May 20–30, 2003, after the Riyadh compound bombings and the Casablanca bombings. According to Tom Ridge: "The U.S. Intelligence Community believes that Al Qaeda has entered an operational period worldwide, and this may include terrorist attacks in the United States."
* December 21, 2003 – January 9, 2004, citing intelligence information suggesting large-scale attacks around the holiday season.
In addition, the alert has been raised to High on a select or partial basis three times:
* August 1 – November 10, 2004, for specific financial institutions in northern New Jersey, New York, and Washington, D.C., citing intelligence pointing to the possibility of a car or truck bomb attack, naming specific buildings as possible targets.
* July 7, 2005 – August 12, 2005, for mass transit systems only. The DHS secretary announced the level after the 7 July 2005 London bombings despite the absence of "specific, credible information suggesting imminent attack" in the United States.
* August 10, 2006 – present, for all domestic airline flights and all international flights to or from the United States, with the exception of flights from the United Kingdom to the United States. Flights from the United Kingdom to the United States had been under a Severe alert, but were downgraded to High on August 13, 2006.
* March 12 – September 10, 2002
* September 25, 2002 – February 6, 2003
* February 28 – March 16, 2003
* April 17 – May 20, 2003
* May 31, 2003 – August 1, 2004
* November 10, 2004 – July 8, 2005
* August 12, 2005 – present (excluding domestic and international flights)
Low (Green) and Guarded (Blue)
The threat level has never been lowered to Low (Green) or Guarded (Blue).
Hearings on ending Saturday delivery for the U.S. Postal Service start July 12, and rate increases are also in the offing.