Chicago Bears fly flags at half staff to honour the anyone that died during the service to their city or country, most recently this was done to honour the death of 19 fire fighters that died.
Traditionally this is when someone dies
To mourn the death of a public official.
The US government flag flying flags at half-staff on April 15, 2014 is in honor of Sergeant Timothy Wayne Owens who died during the Fort Hood shootings.
Only the President or a state Governor can order when and for how long the American flag is to be flown at half-staff on government property.
Running Back/ Half Back
We were told the son of a business man who owns KFC and Taco Bell was killed. The flags at these locations (N1) were at half staff.
In the United States the flags are flown half-staff by order of the President for any amount of time he wishes. However by law 30 days after the death of a current or former president and 10 for vice presidents.
By convention flags are flown at half mast during a period of official mourning (e.g the death of a national leader or a for national tragedy). It is a mark of respect.
President William Jefferson Clinton
For State Government buildings the Governor can and in Michigan every time a Michigan citizen loses their life in the Iraq or Afghanistan Military conflict the flags are ordered Half Staff . It depends on whether you are talking Federal buildings
In memory of someone who has died that has served the country or done something nationally recognized. An event like 9-11 also can have a half mast flags.