Today, in accordance with the President's proclamation, all flags on public buildings and grounds should be be lowered to half-staff in remembrance and as a mark of respect for the victims of the senseless act of violence perpetrated on March 22, 2021, in Boulder, Colorado, and remain at half-staff until sunset, March 27, 2021. This includes any flags flying with the US Flag.

Proclamation can be viewed here: https://www.whitehouse.gov/briefing-room/presidential-actions/2021/03/23/a-proclamation-honoring-the-victims-of-the-tragedy-in-boulder-colorado/ 

If you fly an outdoor flag that cannot be lowered, respect can be shown by tying a black ribbon or cord at the top of the staff.