On the battlefield, the roar of helicopter blades paired with a red cross is salvation for wounded troops and civilians.

But the thwomping blades of military helicopters, including one with red cross markings, were part of a low-flying show of force over Washington’s streets Monday night — an incident now under investigation.

Numerous videos on social media showed an unarmed Lakota medevac helicopter hovering over demonstrators. Its red cross markings, visible on the aircraft’s belly and side, was flown by the Washington D.C. Army National Guard. At least two helicopters roared overhead, sending people and debris flying.

The use of a helicopter with red cross markings was an abuse of global norms that could help erode its neutral symbolism, military justice experts said.

“This was a foolish move,” said Geoffrey Corn, a former Army lawyer and professor at the South Texas College of Law in Houston. “The symbolic significance of the red cross is pervasive: It denotes a ‘noncombatant’ function of the armed forces.”

The National Guard has been used to augment police across the country, defense officials have insisted, including 18,000 Guard members in 29 states and the District mobilized in response to unrest after the death in police custody of George Floyd in Minneapolis last week.

The use of a helicopter’s rotor wash, the downward rush of air from its rotors, is a common military tactic to incite fear, disperse crowds and warn of other capabilities, like rockets and guns, said Kyleanne Hunter, a former Marine Corps pilot who flew Cobra attack helicopters in Iraq and Afghanistan.

Hunter, now a senior adjunct fellow at the Center for a New American Security in Washington, said she flew low over civilian vehicles to disperse them ahead of convoys in Iraq.

And after reviewing videos of the helicopter, she concluded that it flew far too low to be on a surveillance mission.

“You add more military equipment, you get more military tactics,” she said. “It’s to provoke and incite rather to protect and serve.”

One helicopter buzzed protesters from a height nearly level with three- and four-story buildings in the Chinatown area, ripping tree limbs away. Rotor wash hurled glass from broken windows like shrapnel. Protesters and a few remaining journalists scattered to the sides of the street to seek shelter beneath awnings, or went sprinting away, shouting “I can’t hear!” and “I can’t see!”

Many closed or covered their eyes to protect from flying debris and ran blind, causing them to stumble and fall before they scrambled back up.

Maj. Gen. William J. Walker, commander of the D.C. National Guard, has directed an investigation into the “low-flying maneuver,” a spokesman said Tuesday evening. The decision to use the maneuvers was authorized by the highest levels of that command, the New York Times reported.

Guard members were among the local and federal law enforcement officers that cleared out Lafayette Square on Monday to allow President Trump a photo op at a nearby church, but they did not have, and did not fire, tear gas or rubber bullets, defense officials said.

But the use of medevac helicopters during demonstrations after curfew stunned justice experts, who said the red cross symbolizes mercy.

“Misuse of the red cross symbol is prohibited even during peacetime by the First Geneva Convention, to which the U.S. is a party,” said Rachel E. VanLandingham, a former Air Force attorney and professor at the Southwestern Law School in Los Angeles.

Linking the symbol with law enforcement, VanLandingham said, can weaken its “effectiveness as signifying medical and humanitarian assistance, a symbol of trust that is needed to give those vehicles with that symbol needed access where they are needed during armed conflict.”

It is unclear whether the helicopter was one of few available. VanLandingham said it might have been prudent to consider covering up the red crosses before flight.

The use of the helicopter also may violate Army regulations, Corn said, including domestic operations that outline the use of medical resources for “the evacuation of patients, movement of medical supplies and personnel, and support of search and rescue activities.”

Flying low in urban areas presents numerous risks to the pilots, crew, aircraft and people on the ground, Hunter said, factors that are typically considered before flying such a mission.

Helicopters are not as aerodynamic as airplanes, so in the event of a malfunction, pilots need to assess their altitude, speed and weight before calculating what to do. One solution, Hunter explained, is to rotate the aircraft so air can move over the blades — an attempt to control the helicopter into a crash landing.

But the presence of protesters on the ground, buildings on all sides and a low altitude would make such a maneuver “virtually impossible” in the event of engine failure, Hunter said. The best a pilot could hope for at that altitude, she said, is minimizing injuries on the ground.

Hannah Natson contributed to this report.