WASHINGTON — Top Pentagon officials ordered National Guard helicopters to use what they called “persistent presence” to disperse protests in the capital this week, according to military officials. The loosely worded order prompted a series of low-altitude maneuvers that human rights organizations quickly criticized as a show of force usually reserved for combat zones.
Ryan D. McCarthy, the Army secretary and one of the officials who authorized part of the planning for the helicopters’ mission Monday night, said on Friday that the Army had opened an investigation into the episode.
Two Army National Guard helicopters flew low over the protesters, with the downward blast from their rotor blades sending protesters scurrying for cover and ripping signs from the sides of buildings. The pilots of one of the helicopters have been grounded pending the outcome of the inquiry.
The high-profile episode, after days of protests in Washington — some of which turned violent — was a turning point in the military’s response to unrest in the city. After days of operating on the periphery of the crowds, National Guard forces suddenly became a focus of the controversy over the military’s role in urban law enforcement.
Military officials said that the National Guard’s aggressive approach to crowd control was prompted by a pointed threat from the Pentagon: If the Guard was unable to handle the situation, then active-duty military units, such as a rapid-reaction unit of the 82nd Airborne Division, would be sent into the city.
Senior Pentagon officials, including Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, were trying to persuade President Trump that active-duty troops should not be sent into the streets to impose order, and that law enforcement and National Guard personnel could contain the level of unrest.
On Monday night, both Mr. McCarthy and the Army’s chief of staff, Gen. James C. McConville, pressed Maj. Gen. William J. Walker, the commanding general of the District of Columbia National Guard, to increase his forces’ presence in the city, according to a senior Defense Department official.
An Army official declined to comment, saying that the investigation was continuing.
The episode has stirred outrage among lawmakers. “What we saw on Monday night was our military using its equipment to threaten and put Americans at risk on American soil,” said Senator Tammy Duckworth, an Illinois Democrat and former Army Black Hawk pilot.
Documents obtained by The New York Times show that planning for the National Guard mission included oversight by Mr. McCarthy and General McConville. The operation had been reviewed by a judge advocate team — military lawyers — before aviation units were instructed to apply “persistent presence.” These types of maneuvers are well known to Mr. McCarthy, who served in the Army’s elite Ranger Regiment during the opening operations of the war in Afghanistan.
The episode, which occurred about three hours after a 7 p.m. curfew in the capital went into effect on Monday, began when a Black Hawk helicopter, assigned to the District of Columbia National Guard, began a low and slow pass over a group of roughly 200 peaceful protesters in the Chinatown neighborhood.
The downward force of the helicopter’s rotor blades snapped a small tree, with debris almost hitting several people. The second helicopter tried a similar maneuver. Roaring overhead, the Lakota, adorned with a red-and-white cross denoting its medical affiliation, hovered over the crowd, staying at rooftop level, blowing debris and sending protesters scattering.
The red cross with white background is a “universally recognized symbol of medical aid and is protected under the Geneva Conventions,” Human Rights Watch said in a report Friday. “Its misuse is prohibited under the conventions and it has no place in a ‘show of force’ or to forcibly disperse protesters.”
“The wind speeds created by a low-hovering helicopter can lift objects and cause serious damage, potentially leading to injury or death,” the report said. “These risks are amplified in congested urban environments, where the consequences would be exceptionally dangerous if something were to go wrong.”
On Wednesday, Defense Secretary Mark T. Esper said that he was told that the helicopters had been asked by law enforcement to look at a checkpoint to “see if there were protesters around.”
“We need to let the Army conduct its inquiry and get back and see what the facts actually are,” Mr. Esper told reporters. The District of Columbia National Guard is the only unit of the Guard that reports directly to the president because of the capital’s unusual political status — it has no governor, who usually commands the units.
A District of Columbia National Guard spokeswoman declined to comment, citing the open investigation.
During the operation Monday night, the helicopters followed the crowd through several well-lit intersections and repeatedly hovered over protesters for close to an hour.
People at the scene expressed their disbelief and fear. One protester, asked by a friend if he wanted to stay out later, responded curtly that he was just “trying not to die.”
There is no formal training for the type of maneuvers conducted Monday night, said one military official with direct knowledge of the episode, so any guidance about “persistent presence” is left to the interpretation of the pilots.
The use of a medical evacuation helicopter, the official added, appeared to result from the fact that command levels of the District of Columbia National Guard did not realize that the majority of the Lakota helicopters available for law enforcement missions are deployed to the Texas border for Customs and Border Protection missions there to halt illegal immigration.
While many Army aviation units have the Red Cross symbol in a detachable form, by way of magnets, the District of Columbia National Guard has the cross painted on the airframes of its helicopters since they are so often involved in a patient transfer program that moves people among routine, urgent and critical care facilities in the Washington area, the official added.
The unit responsible for Monday’s episode performed a lifesaving transfer mission the next day, transporting a deteriorating patient from Ft. Belvoir’s community hospital in Virginia to Walter Reed National Military Medical Center in Bethesda, Md., the official said. On Friday, The Washington Post reported that all District of Columbia National Guard helicopter operations had been suspended pending the results of the investigation, although it was unclear if that affects medical patient transfers.