WASHINGTON — Saddam Hussein had been gone just a few weeks, and American forces in Fallujah, west of Baghdad, were already being called unwelcome invaders. One of the first big anti-American protests of the war escalated into shootouts that left 18 Iraqis dead and 78 wounded.

It would be a familiar scene in Iraq’s next few years: Crowds gather, insurgents mingle with civilians. Troops open fire, and innocents die. All the while, according to internal military correspondence obtained by the Associated Press, American commanders were telling Washington that many civilian casualties could be avoided by using a new nonlethal weapon developed over the past decade.

Military leaders repeatedly and urgently requested — and were denied — the device, which uses energy beams instead of bullets and lets soldiers break up unruly crowds without firing a shot.

It’s a ray gun that neither kills nor maims, but the Pentagon has refused to deploy it out of concern that the weapon itself might be seen as a torture device.

Perched on a Humvee or a flatbed truck, the Active Denial System gives people hit by the invisible beam the sense that their skin is on fire. They move out of the way quickly and without injury.

On April 30, 2003, two days after the first Fallujah incident, Gene McCall, then a top scientist at Air Force Space Command in Colorado, typed a two-sentence e-mail to General Richard Myers, chairman of the Joint Chiefs of Staff.

“I am convinced that the tragedy at Fallujah would not have occurred if an Active Denial System had been there,” Mr. McCall told General Myers, according to the email obtained by AP. The system should become “an immediate priority,” Mr. McCall said.