Every few years since 2001, the military has invited reporters to get zapped by a “heat ray.” The dozens who signed up all came away with a similar conclusion: The weapon, officially called an Active Denial System, is brutally painful.

“It felt as if I had opened a furnace with my face too close and been hit by a wall of scorching heat,” wrote Philip Sherwell for the Sunday Telegraph in 2007, calling the pain “intolerable.” Five years later, Wired’s Spencer Ackerman said it felt like he’d “been exposed to a blast furnace.”

The pain faded quickly, which was the point. The military has spent millions on the weapon hoping it could become a less-lethal crowd control option. Yet, other than tests on luckless journalists and military volunteers, the devices have never been used.

Trump administration officials have reportedly sought twice to change that, first in a 2018 bid to deploy heat rays against migrants at the border and then again this summer — as a whistleblower said in testimony provided to The Washington Post on Wednesday — against D.C. protesters.

That revelation has sparked alarm among civil rights advocates and critics, who point out that there’s a reason American soldiers have never used the heat rays: The weapons have been dogged by safety concerns and ethical worries.

“There is nothing ‘routine’ about inquiring about the availability of a heat ray to use against American citizens exercising their first amendment rights,” said David Laufman, an attorney representing Major Adam DeMarco, the D.C. National Guard whistleblower.

The backstory of the heat ray starts in the mid-1990s at Air Force research labs in Texas and New Mexico. The impetus was the fierce 1993 gun battle in Mogadishu that killed 19 U.S. soldiers and hundreds of Somalis, which led strategists to ponder a less deadly option for forces cornered in an urban area.

By the time the Pentagon unveiled its prototype heat ray in 2001, it had been tested on 72 volunteers at Brooks Air Force Base in San Antonio, the Associated Press reported at the time. The weapon, which looks like a satellite dish perched on a vehicle, showed promising results.

“It’s the kind of pain you would feel if you were being burned,” Rich Garcia, a spokesman for the Air Force Research Laboratory in New Mexico, told the AP. “It’s just not intense enough to cause any damage.”

The effect comes from a gyrotron, which creates heat by pushing energy through a magnetic field, similar to a microwave, Ackerman reported in Wired. But unlike the kitchen appliance, the weapon generates millimeter waves that only penetrate 1/64th of an inch into the skin — enough, in theory, to hurt like crazy without leaving burns.

Encouraged, the military spent millions developing and testing the device, zapping thousands of military volunteers. Yet, when the first serious demand came to deploy it into combat during the Iraq War, military leaders demurred.

Officials worried it hadn’t been tested enough, the Orlando Sentinel reported in 2007. Still facing massive backlash over revelations of prisoner abuse in Abu Ghraib, leaders also thought the heat ray might be seen as a torture device.

Those safety concerns ticked up in 2008, when operators accidentally used a full-power setting on an airman, who suffered second-degree burns. (Military officials noted this was the only recorded serious injury among thousands of tests.)

But there were other logistical problems. As Wired reported, the heat ray didn’t work well in rain, snow or dust, and at that time, it took 16 hours to set up, which made it useless for responding to unexpected turmoil.

Though the heat ray was actually shipped to Afghanistan in 2010, it was recalled within weeks and never used. The commander there, Army Gen. Stanley McChrystal, worried the Taliban would make propaganda hay of the futuristic weapon by claiming the United States was microwaving Afghans and giving them cancer, according to Wired.

In fact, ethical concerns have long shadowed the device. When it first debuted in 2001, Human Rights Watch adviser William M. Arkin wondered how it might affect children or pregnant women who happened to be in a crowd.

“We have developed a nonlethal weapon which causes pain. What happens when someone continues to walk toward the source of the high-powered microwave? What happens when panic ensues in a crowd as a result?” he asked.

Others contended it would inevitably be used as a torture device.

“It seems fundamentally a weapon that’s designed to create a great deal of pain and fear,” Douglas A. Johnson, then the executive director of the Center for Victims of Torture, told the Sacramento Bee in 2004. “Once this kind of technology is available and there’s a perception that it’s safe and nonlethal, it seems like a natural device to be used in interrogations.”

Between the ethical, safety and logistical concerns, the heat ray has been sidelined since its brief foray into Afghanistan.

Critics lashed out at the Trump administration on Wednesday after federal officials suggested using it against civilian protesters in D.C.

“Our government shouldn’t be conspiring to use heat rays against us for exercising our constitutional rights,” the American Civil Liberties Union tweeted on Wednesday.