Using a pair of thumb-controlled joysticks, a Dallas police officer guided a robot loaded with a pound of C-4 plastic explosive toward cop-killer Micah Xavier Johnson and blew him up.

The unprecedented decision to remotely blast Johnson, who had killed five officers in one of the worst ambushes against U.S. law enforcement in modern history, was widely praised as an innovative way to eliminate a threat without risking more officers’ lives. Police said they came up with the deadly plan in less than 20 minutes, after Johnson said that “the end was coming” and negotiations with him broke down.

Now their use of a robot is prompting debate about the role of remote-controlled robots in law enforcement and whether their use to deliver lethal force should be left to the discretion of individual police departments or regulated by state or federal governments.

“We’ve crossed a new frontier, and we look out and we see an absence of law and policy,” said Peter Singer of New America, who has written extensively about technology, security and robotics.

That void, some worry, has the potential to lead to overuse of machines that can be used to injure or kill suspects.

An explosives robot is prepared and sent into the Discovery Channel networks building where police shot and killed a gunman who took hostages in Silver Spring, Md., on Wednesday, Sept. 1, 2010. (Jacquelyn Martin/AP)

“Technology can change things,” said Jay Stanley of the American Civil Liberties Union. “When things become easier, they tend to become overdone, and sometimes you need to reassess rules.”

But police officials and others interviewed said that robots were simply another tool in the police arsenal, and their use was already subject to strict laws and regulations about lethal force.

“Technology cannot override the legal standards governing police use of deadly force,” said Chuck Wexler, executive director of the Police Executive Research Forum, a nonprofit research and policy group.

Americans have grown accustomed to a climate of fast-changing technology and the rise of robotics, from the widespread use of unmanned aerial drones to the rise of self-driving cars — both of which have caused passionate debate over how they should be regulated.

The use of drones by the U.S. military has long been the subject of furious ethical and legal wrangling. But the explosion of growth in civilian drones, including increasing use of unmanned aircraft by police departments, has also led to increasing government regulation. At least 41 states have considered drone legislation this year alone, covering everything from operating drones over prisons in Wisconsin to banning their use in hunting in Idaho.

Several states have passed or proposed regulations governing how police can use drones, from requiring that police obtain warrants before conducting aerial surveillance to limits on how long departments can retain the data they record, according to Arthur Holland Michel, co-director of Bard College’s Center for the Study of the Drone.

A Remotec Andros F6A robot, similar to the model used by Dallas police to kill the gunman who ambushed their officers, is used by Fairfax County police to move a simulated pipe bomb to a safe area during a training exercise. (Lt. Luke Durden/Fairfax County Police Department)

But officials and analysts interviewed said they knew of no state or local legislation or regulation that had been proposed to cover ground based-robots.

It is unclear precisely how many U.S. law enforcement agencies have robots, but law enforcement experts said every major department, plus many smaller departments have them. The FBI, sheriff’s departments and federal and local agencies also have robots. Many come from a federal program that transfers excess U.S. military equipment to law enforcement agencies.

The robots, which have been used by police since the 1990s, are typically outfitted with cameras that allow police to peer into situations that may be dangerous for officers.

Partly, analysts said, the lack of regulation is because the public is not widely aware that police departments have robots. Dallas Police Chief David O. Brown’s announcement that Johnson had been killed by a police robot caught many people by surprise.

The Dallas robot was Northrop Grumman’s Remotec Andros Mark V-A1, which is a four-wheel platform, about 3.5 feet wide and 4 feet long, and weighing 790 pounds. It is fitted with a camera and an extendable arm that can carry payloads of up to 60 pounds.

Brown has said officials came up with the plan to use the explosives against Johnson, who had fled into a college building, after negotiations failed. The chief said Johnson “had delusions and was committed to killing officers.”

The chief said he told his team, “don’t bring the building down, but that was the extent of my guidance.”

“This was not an ethical dilemma. I would do it again,” Brown said of using lethal force, adding: “I would use any tool necessary to save our officers’ lives.”

Brigitt Keller, executive director of the New York-based National Police Accountability Project, was not convinced. She said she opposes the use of weaponized robots by police, fearing the tactic could become more widely used.

“Was there really no other option than to use a robot to blow this person up?” she said. “It’s an emotional situation, I understand that. But even a person who killed five police officers deserves due process. . . . It’s important that the Constitution applies, that somebody cannot just be summarily executed.”

Police have found other innovative uses for robots:

After police shot and killed a gunman with a live bomb strapped to his chest who had taken over the Discovery Channel building in Silver Spring in 2010, Montgomery County police used a remote-controlled robot to defuse the explosive, Police Chief Tom Manger said.

In 2007, Fairfax police used a robot to deliver a cellphone and a bottle of water to a gun-wielding man who had taken over a public bus. The phone allowed police to negotiate with the man, who later surrendered.

Dallas police used a robot last year to confirm the death of a suspect who had taken cover inside an armored van after shooting at police headquarters. Police snipers had shot through the window of the van, striking the man, but officials didn’t want officers to approach the vehicle because the suspect had said he had explosives.

Singer said he was aware of only one other instance where a robot was used as an offensive lethal weapon, and that was by the U.S. military. He said soldiers in Iraq in the mid-2000s used duct tape to affix a land mine to a surveillance robot and maneuvered it into position to blow up an armed insurgent holed up in an alley.

Police robots have vastly increased under a federal program to transfer excess equipment from the U.S. military to local law enforcement agencies. Since President Clinton signed the program into law in the 1990s, more than $6 billion worth of equipment has been turned over, according to the Defense Logistics Agency.

Much of the equipment transferred has come since the post-9/11 wars in Afghanistan and Iraq. Since 2003, 280 law enforcement agencies have obtained at least 987 reconnaissance and bomb-disposal robots from the military, according to Holland Michel.

He said many more have been purchased commercially; Dallas police declined to comment on where they obtained their robot, except to say that it cost $151,000 in 2008.

Although Holland Michel stopped short of advocating new guidelines for police departments, he said the Dallas incident highlighted that “some level of policy discussion, and some policy,” will be needed to regulate the use of robots as lethal weapons.

“This is really uncharted territory,” he said.

Singer said that even before the incident in Dallas, companies at industry trade shows had been promoting robots outfitted with “less than lethal” weapons, such as tear gas, stun guns or guns that fire rubber bullets.

North Dakota is the only state that allows aerial drones to be fitted with “less than lethal” weapons, analysts said.

Robert Taylor, a professor of criminology at the University of Texas at Dallas, said he believed there was no need for new national standards or regulation.

He said Dallas police concluded that lethal force was legally justified against Johnson, and that they made a decision that eliminated his threat without endangering more officers.

“It’s no different than when a commander on a scene says, ‘I’ve got snipers on rooftops’ and giving them the order to take them out,” Taylor said. “I don’t think anybody in policing is advocating that this should be standard practice. They’re simply saying in this condition this was the best way to save lives.”

Some law enforcement agencies seem keenly aware of the issues surrounding robots and drones and have sought guidance in their use.

Capt. Tom Madigan of the Alameda County (Calif.) sheriff’s department said his department has consulted with privacy rights groups, local prosecutors, pilots and other police departments, and has held public hearings to explain the use of robotics to local residents.

But Singer said huge differences in the quality of training and performance among police departments argues for national standards for the use of robots.

He noted that before the shooting, Dallas police officers in regular uniforms had been protecting a Black Lives Matter march protesting the killing of black men by police in Louisiana and Minnesota. A few days later, police in Baton Rouge were in full battle gear treating similar protesters “as the enemy in a quasi-military manner.”

“Some police departments do it well, and some do it really badly, and it’s having a poisonous effect across society,” Singer said. “Do we want that to be the case with robotics?”

Keith L. Alexander contributed to this report.