DOVER-FOXCROFT — A state trooper who shot and killed a gunman during a standoff initially was unaware that the man was communicating with law enforcement on a different radio channel and fired the fatal shot when the sheriff walked toward the gunman, the attorney general’s office said Monday.

Trooper Jon Brown fired three shots at Michael Curtis without realizing Curtis, a firefighter, was communicating with the sheriff on a channel used by the fire department, according to a report by the attorney general’s office.

Brown had become aware of the communications minutes later when the sheriff walked out into the open and moved toward the gunman, prompting the trooper to fire the fatal shot, the report said.

Attorney General William Schneider concluded that the trooper met the legal requirement for using deadly force because he thought the sheriff was under imminent threat.

Curtis, who had been drinking, had ignored orders to drop the .40-caliber semiautomatic handgun that he’d used earlier to kill his wife’s ex-husband at work at the Hilltop Manor nursing home on Nov. 29, the report said.

The standoff took place at a local fairground where the 46-year-old Curtis began communicating with law enforcement officials on a two-way radio on a channel used by fire officials.

Sheriff John Goggin offered assurances, unbeknownst to Brown, that Curtis would not be harmed before Brown shouted for the gunman to “show your hands!” and fired the initial volley. That prompted Curtis to exclaim, “I want help, but nobody seems to want to help me, all they want to do is shoot me,” the report said.

Afterward, Goggin walked toward Curtis, despite concerns voiced by several officers. Brown yelled several warnings for the sheriff to “get out of the field” before firing the fatal shot, the report said.