Why is the american flag still being flown at half-staff?

The United States flag at half-staff is to consider when the whole nation is in mourning. These periods of mourning are proclaimed either by the president of the United States, for national remembrance, or the governor of a state or territory, for local remembrance, in the event of a death of a member or former member of the federal, state or territorial government or judiciary. It means a very important person/ a tragedy has happened to our nation. 

The inuted states of america