From a Black Hawk helicopter circling Salt Lake City to Predator drones cruising high above Minneapolis, military aircraft have been used to monitor, and in some cases respond to, recent protests against police brutality and its impact on black Americans.

The Trump administration’s use of Predator drones to monitor civil unrest in Minneapolis has raised questions about how military-grade surveillance equipment is being used to spy on protests.

In a letter Tuesday, around three dozen Democratic House members called for protest surveillance to end, The Wall Street Journal reported.

“Americans have a healthy fear of government surveillance that started at the founding of our country and has continued to modern times,” the letter says. According to the representative’s letter, drones were also used to monitor protests in San Antonio and Detroit, reported The Wall Street Journal.

A U.S. Customs and Border Protection drone was first spotted over a police brutality protest on May 29 — four days after the death of George Floyd in police custody. A white Minneapolis police officer has been charged with Floyd’s murder.

Senior policy analyst Bill Easly II at Americans for Prosperity — a conservative political action group supported by Charles and David Koch — said “it’s disturbing to see tools built to gather military intelligence being used to watch U.S. citizens,” according to The Wall Street Journal.

Vice reported that open-source flight tracking software showed that a drone took off from Grand Forks Air Force Base in North Dakota and flew over protesters in Minneapolis at 20,000 feet before returning back to the air base.

The U.S. Customs and Border Patrol uses drones to monitor the border but must stay within 100 miles of that border, according to Vice.

“This Administration has undermined the First Amendment freedoms of Americans of all races who are rightfully protesting George Floyd’s killing,” the letter says.

Manned aircraft have also been used to monitor and, at times, intimidate protesters the last two weeks.

In Washington, D.C., Army National Guard helicopters were used to scare protesters by flying directly overhead, sometimes just around 100 feet, The Hill reported.

Several videos show a helicopter — what the Army calls a “Lakota” and marked for medical evacuation missions — directing its prop blast down onto protesters in Washington on the evening of June 1.

A National Guard Black Hawk helicopter conducted similar low-flying maneuvers, and at least one pilot has been grounded — not allowed to fly — while the incident is being investigated, The New York Times reported.

The air pressure created by the helicopters’ rotors — what Army leadership referred to the as “persistent presence” — was strong enough to snap a small tree and tear signs off buildings, according to The New York Times. The use of military aircraft to intimidate an enemy in combat in this fashion is referred to as a “show of force” in the military.

Lawmakers have criticized the use of military force to clear the capital’s streets last Monday.

“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, according to The New York Times. Duckworth — a Democrat from Illinois — is a former Army pilot who lost her legs in Iraq after her Black Hawk was attacked by a rocket-propelled grenade.

In Salt Lake City, a Utah National Guard Black Hawk helicopter flew above a crowd of thousands of protesters on Saturday, May 30, and circled over the state capitol and other areas where protests continued during a citywide curfew that lasted for days.