USFlag.org: A website dedicated to the Flag of the United States of America - Flag Etiquette
FLYING THE FLAG AT HALF-STAFF: The pertinent section of the Flag Code says, "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 possesion, 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 orders, or in accordance with recognized customs or practices not inconsistent with law.
In the event of the death a present or former official of the government of any State, territory, or possession of the United States,
the Governor of that state, territory, or possession may proclaim that the National flag shall be flown at half-staff." The code also includes other related details including the specific length of time during which the flag should be displayed at half-staff, in the event of the death of a "principal figure"(e.g., 30 days for the death of a sitting or former President, 10 days for the death of a sitting Vice-President,etc.).
Would you like to know when the President has ordered the Flag flown at half-staff? Search the press releases:
http://library.whitehouse.gov/Search/Query-PressReleases.html
Protocol for Flying MD's Flag at Half Staff Maryland State Flag