The plan for a pandemic drone didn’t last long in Westport, Conn.

Within days in April, the police department of the coastal town reversed course on using drone-mounted cameras to scan crowds for fevers and coughs. Feedback from some of the 28,000 residents was quick, Lt. Anthony Prezioso said. “This is not really a time to divide people.”

At least 40 law enforcement agencies across the country have used drones in the past few months for coronavirus-related purposes, said a Stateline review.

But drones raise the question of what surveillance the public will accept in a tense time. The new measures — monitoring social distance, scanning crowds, testing temperatures — also worry civil liberties advocates and some in the drone industry.

“This is not a time to be, in my opinion, ramrodding the aircraft into the air,” said Matt Dunlevy, who owns SkySkopes, a Grand Forks, N.D.-based drone company.

The Federal Aviation Administration requires drone pilots to be certified and keep drones in sight and not above people, with some exceptions. As long as its rules are followed, the agency doesn’t regulate how a drone is used. “I think it’s so cool that all these agencies are doing stuff,” said Ian Gregor, a spokesperson for the FAA. “We had no idea we’d be seeing this kind of public health agency use.”

At least 18 states require law enforcement agencies to get a search warrant to use a drone for surveillance or for a search, the group said. But those laws leave room for uses such as crowd surveillance.

“Either of those use cases don’t violate the Fourth Amendment because people in public places don’t have a reasonable expectation of privacy,” said Gregory S. McNeal, a professor of law and public policy at Pepperdine University in California. “The other bigger question is whether we as a society want drones flying around as the enforcers of these bureaucratic rules. … That’s less of a legal question and more of a social acceptance question.”

Civil liberties advocates worry that the pandemic will push law enforcement to go to extreme lengths to adopt fast-moving technology. “Do we really want to live in a state where we have this ever-present eye in the sky that’s collecting information about individuals, about their private, personal health?” said Kara Gross of the American Civil Liberties Union of Florida.

Matthew Guariglia, a policy analyst with Electronic Frontier Foundation, a nonprofit digital rights group, said he was skeptical of the accuracy of thermal imaging cameras and questioned their use by law enforcement rather than health officials. But even sending a drone with loudspeakers toward a crowd gave him pause. “It normalizes policing and governance by robot,” he said.

And, he warned, even if a technology is adopted under the guise of short-term use, it’s less likely to be removed after the crisis.

“It might be for policing social distancing now, but in seven or eight months it might be sending that drone over protests against a presidential election,” Guariglia said.