Why is the flag at half mast today?

Flags are typically flown at half-mast (or half-staff) as a sign of mourning, respect, or remembrance following a significant event such as the death of a national leader or public figure, a tragic event, or a national holiday or observance.
In the United States, the flag is flown at half-mast (or half-staff) on certain days to commemorate or honor specific events, as well as to honor the deaths of prominent individuals. Some of the occasions when the US flag is flown at half-mast include:
Memorial Day: On Memorial Day, the flag is flown at half-mast until noon to honor the men and women who died while serving in the US military.
Peace Officers Memorial Day: On May 15th of each year, the flag is flown at half-mast to honor the law enforcement officers who died in the line of duty.
National Fallen Firefighters Memorial Service: The flag is flown at half-mast on the first Sunday in October to honor the firefighters who died in the line of duty.
Pearl Harbor Remembrance Day: On December 7th of each year, the flag is flown at half-mast to honor the victims of the attack on Pearl Harbor.
Patriot Day: On September 11th of each year, the flag is flown at half-mast to honor the victims of the September 11th terrorist attacks.
The death of a President or former President of the United States (number of days specified by the President)
The death of a Vice President, Chief Justice, or retired Chief Justice of the United States (number of days specified by the President)
The death of a member or former member of Congress (on the day of death and the following day)
Flag Day (June 14th)