On Tuesday, President Joe Biden ordered that the flags at the White House, federal buildings and military posts be flown at half-staff “as a mark of respect for the victims of the senseless acts of gun violence” at a Fourth of July parade in Highland Park, Illinois.

The White House issued the proclamation Tuesday, the day after news broke of the shooting that killed six people and wounded dozens more.

The president has the power to order flags lowered to half-staff after tragic events. Flags will remain at half-staff until sunset on July 9, according to the proclamation.

Around 10 a.m. on Monday, a gunman, perched on a rooftop, used a high-powered rifle to fire upon a crowd of Fourth of July paradegoers, officials said.

After the shooting, an hourslong manhunt ensued. That ended in a traffic stop and brief chase when police arrested a person of interest in the case.

Police said he had planned the attack for weeks and on Monday, was able to access the roof of a business near the parade route and fired over 70 rounds into a panicking crowd.

Officials said there was no indication that anyone else acted with him. A motive is currently unknown and charges are still pending.