The tradition is to fly the flag at half staff for 30 days upon the death of a former president. Lowering the flag for civilian deaths is relatively rare and there is no guideline for the length of time it should fly at half staff...regardless of the tragedy.
http://www.classbrain.com/artfree/publish/article_195.shtml--SNIP--
In the early days of our country, no regulations existed for flying the flag at half-staff and, as a result, there were many conflicting policies. But on March 1, 1954, President Dwight Eisenhower issued a proclamation on the proper times.
The flag should fly at half-staff for 30 days at all federal buildings, grounds, and naval vessels throughout the United States and its territories and possessions after the death of the president or a former president. It is to fly 10 days at half-staff after the death of the vice president, the chief justice or a retired chief justice of the United States Supreme Court, or the speaker of the House of Representatives. For an associate justice of the Supreme Court, a member of the Cabinet, a former vice president, the president pro tempore of the Senate, the majority leader of the Senate, the minority leader of the Senate, the majority leader of the House of Representatives, or the minority leader of the House of Representatives the flag is to be displayed at half-staff from the day of death until interment.
The flag is to be flown at half-staff at all federal buildings, grounds and naval vessels in the Washington, D.C., area on the day and day after the death of a United States senator, representative, territorial delegate, or the resident commissioner from the Commonwealth of Puerto Rico. It shall also be flown at half-staff on all federal facilities in the state, congressional district, territory, or commonwealth of these officials.
Upon the death of the governor of a state, territory or possession, the flag shall be flown at half-staff on all federal facilities in that governor's state, territory or possession from the day of death until interment.
The president may order the flag to be flown at half-staff to mark the death of other officials, former officials, or foreign dignitaries. In addition to these occasions, the president may order half-staff display of the flag after other tragic events.
The flag should be briskly run up to the top of the staff before being lowered slowly to the half-staff position.
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theProdigal