The flag is to be flown at half staff in mourning for designated, principal government leaders and upon presidential or gubernatorial order.
United States Code Title 4 Chapter 1 — The Flag
§7. Position and manner of display
-m.
“...By order of the President, the flag shall be flown at half-staff
upon the death of principal figures of the United States Government
and the Governor of a State, territory, or possession,
as a mark of respect to their memory.
In the event of the death of other officials or foreign dignitaries,
the flag is to be displayed at half-staff according to Presidential instructions...”
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http://www.ushistory.org/betsy/flagcode.htm
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