GRAND FORKS, N.D. — The Department of Homeland Security deployed helicopters, airplanes and drones over 15 cities where demonstrators gathered to protest the death of George Floyd, logging at least 270 hours of surveillance, far more than previously revealed, according to Customs and Border Protection data.

The department’s dispatching of unmanned aircraft over protests in Minneapolis last month sparked a congressional inquiry and widespread accusations that the federal agency had infringed on the privacy rights of demonstrators.

But that was just one piece of a nationwide operation that deployed resources usually used to patrol the U.S. border for smugglers and illegal crossings. Aircraft filmed demonstrations in Dayton, Ohio; New York City; Buffalo and Philadelphia, among other cities, sending video footage in real time to control centers managed by Air and Marine Operations, a branch of Customs and Border Protection.

The footage was then fed into a digital network managed by the Homeland Security Department, called “Big Pipe,” which can be accessed by other federal agencies and local police departments for use in future investigations, according to senior officials with Air and Marine Operations.

The revelations come amid a fierce national debate over police tactics and the role that federal law enforcement and military forces should play in controlling or monitoring demonstrations. The clearing of demonstrators from Lafayette Park in Washington for a presidential photo op is still under scrutiny. The Air Force inspector general is investigating whether the military improperly used a reconnaissance plane to monitor peaceful protesters in Washington and Minneapolis this month.

And the National Guard in the District of Columbia has already reached a preliminary conclusion that a lack of clarity in commands led to one of its medical evacuation helicopters swooping low on protesters in the nation’s capital. Renewed calls to demilitarize police work have not only come from criminal justice advocates but also former Republican Homeland Security officials such as Michael Chertoff and Tom Ridge, the first two leaders of the Homeland Security Department, which was created after the Sept. 11, 2001, attacks.

Officials at the Customs and Border Protection base here rejected any notion that their fleet of aircraft had been misused, either to violate privacy rights or intimidate protesters.

“The worst part for me is when we’re made out to be storm troopers,” said David Fulcher, the deputy director for air operations at the National Air Security Operations Center in Grand Forks. “We believe in peaceful protests.”

The aircraft, they said, were used to provide an eagle-eyed view of violent acts and arson. The Predator drone deployed to Minneapolis, like eight other unmanned aircraft owned by Air and Marine Operations, was neither armed nor equipped with facial recognition technology and flew at a height that made it impossible to identify individuals or license plates, according to senior officials here.

“The legend of the Predator — the all-seeing, all-knowing, hover-outside-your-window Predator — it’s just not accurate,” Mr. Fulcher said. “The technology is not there.”

But House Democrats and privacy advocates still worry over the potential dissemination of the footage and the chilling effect that militarized aircrafts could have on peaceful protests.

Earlier this month, Democrats with the House Oversight Committee, including Representatives Carolyn B. Maloney and Alexandria Ocasio-Cortez of New York, Jamie Raskin of Maryland, and Stephen F. Lynch and Ayanna Pressley, both of Massachusetts, protested to Chad Wolf, the acting secretary of homeland security.

“This administration has undermined the First Amendment freedoms of Americans of all races who are rightfully protesting George Floyd’s killing,” the Democrats said in a letter to Mr. Wolf. “The deployment of drones and officers to surveil protests is a gross abuse of authority and is particularly chilling when used against Americans who are protesting law enforcement brutality.”

But Democrats apparently were unaware of the breadth of the agency’s actions. Most of surveillance was done with planes and helicopters. Air and Marine Operations did dispatch drones to two demonstrations — in Minneapolis and in Del Rio, Texas.

The agency’s AS350 helicopters conducted more than 168 hours of surveillance of protests in 13 different cities, the longest stretch being 58 hours over Detroit, according to data provided by Air and Marine Operations. The agency also deployed a Blackhawk helicopter for nearly 13 hours, assisting other federal agencies with surveillance in Washington, D.C. Kris Grogan, a spokesman for Customs and Border Protection, said the agency’s Blackhawk was not one of the helicopters that flew low over the demonstrators and caused panic.

A Cessna single-engine plane conducted nearly 58 hours of surveillance, more than 38 of them over Buffalo. Mark Morgan, the acting commissioner of Customs and Border Protection, said in a tweet this month that the officers manning that plane helped track down suspects who used an S.U.V. to hit local police on the ground.

Most of the requests did not come from local police departments. In Minneapolis, the call came from an agent in Homeland Security Investigations, the branch of Immigration and Customs Enforcement that conducts longer-term investigations into terrorists, weapons trafficking and drug smuggling.

The agent, who was on the ground in Minneapolis and works with Air and Marine Operations regularly, requested the help on May 28 after reports of arson and violence in the area. Air and Marine Operations, which also dispatches drones from Sierra Vista, Ariz., and Corpus Christi, Texas, was not able to send the aircraft until the next day. After about two hours of surveilling, the agent and other law enforcement agencies said it was no longer needed.

“It’s discretionary, but there’s a huge degree of accountability as far as who can say yes or no to deploying these assets,” said Jonathan Miller, the executive director of the National Air Security Operations at Customs and Border Protection.

Air and Marine Operations officials said agency protocol prevents infringement on the right to protest. The drones, which can stay in the air from 12 to roughly 24 hours depending on how much radar equipment is attached, are directed to fly no lower than 19,000 feet. From that height, the “electrical optical-infrared ball” on the drones wouldn’t allow the operators to see faces, eyes or hair color, according to the Department of Homeland Security’s privacy impact assessment for the aircrafts.

But operators can track movements of protesters or looters, direct law enforcement on the ground and see if someone is wearing a backpack or rifle. And stored footage could be accessed later to corroborate investigative findings, such as a witness account that a fire was set at a given time by a small group or the escape route of a suspect.

A live feed of the footage is sent to a mobile operations center, where a group of agents monitor television screens while moving the drone with joysticks. Other federal agents that request a view from the sky can also see the footage on their phones, Mr. Fulcher said.

Mr. Fulcher said the surveillance footage, stored on the aircraft and in control rooms, is overwritten after an average of 30 days by new feeds. But video feeds and radar images sent to “Big Pipe” can also be analyzed by Homeland Security Department intelligence officers. That data may be stored for “up to five years,” according to Homeland Security’s Privacy Impact Assessment. If federal agencies or police departments can prove they need the footage for a criminal investigation, the video can be provided, according to the document and Mr. Fulcher.

The Department of Homeland Security did not say whether any law enforcement agencies had requested footage of the demonstrations.

Jay Stanley, a senior policy analyst at the American Civil Liberties Union, said the aircraft could discourage people from protesting. The concern is not only what the border agency is doing with the aircraft and footage but how future operations could adapt to quickly advancing technology.

“You see an aircraft, you have no idea currently what technologies that aircraft is carrying,” Mr. Stanley said. “There is something militaristic and dominating about a militarized police aircraft hovering over you when you’re out there protesting police abuse.”

Air and Marine Operations recorded more than 92,800 hours of flight time in the fiscal year that ended in September, most of that spent patrolling the border. But the helicopters, planes and drones spent more than 8,000 hours helping law enforcement agencies with search and rescue missions and other criminal investigations.