The helicopter hovered very low and directly above peaceful protestors Monday using its rotor wash to blow debris on them, a dangerous tactic used by the military in war situations to force people to disperse.

The act itself was criticized, as well as the use of a medical helicopter bearing the international symbol of the Red Cross, which in wartime delineates non-combatant and neutral vehicles carrying injured persons and medical personnel.

Defense Secretary Mark Esper said Wednesday that he had ordered an inquiry into the helicopter, which was operated by the Washington DC National Guard, which is overseen by the Pentagon.

Esper said he understood that the helicopter was not on a medical evacuation mission, and that its movements appeared to be "unsafe."

"There are conflicting reports. I think we need to let the army conduct its inquiry and get back and see what the facts are," he told reporters.

Major General William J. Walker, commanding general of the National Guard for the US capital, said he had also ordered an immediate probe.

"I hold all members of the District of Columbia National Guard to the highest of standards," he said.