Presidential Proclamation has been issued:

"As a mark of respect for the victims of the senseless acts of violence perpetrated on April 15, 2021, in Indianapolis, Indiana, by the authority vested in me as President of the United States by the Constitution and the laws of the United States of America, I hereby order that the flag of the United States shall be flown at half-staff at the White House and upon all public buildings and grounds, at all military posts and naval stations, and on all naval vessels of the Federal Government in the District of Columbia and throughout the United States and its Territories and possessions until sunset, April 20, 2021".

See Proclamation at https://www.whitehouse.gov/briefing-room/presidential-actions/2021/04/16/a-proclamation-on-honoring-the-victims-of-the-tragedy-in-indianapolis-indiana/ 

All flags flying with the United States Flag should also be lowered. If you have an outside flag that cannot be lowered, respect can be shown by tying a black cord or ribbon to the top of the mast.