BRENTWOOD, N.H. — The police officer who was gunned down responding to a domestic dispute did everything right but walked into an ambush launched by a gunman who fired from an elevated position of overwhelming advantage, police said Monday.

Police also said the fire that consumed the house on a quiet suburban street after Officer Stephen Arkell was killed May 12 was intentionally set by the killer, 47-year-old Michael Nolan. A spokesman for the Bureau of Alcohol, Tobacco, Firearms and Explosives said it appears Nolan set several fires on both floors throughout the house; after bullets he fired pierced a propane gas line, the flammable gas exploded, leveling the two-unit house.

The street has since been reopened, and the burned hull of the house remains, the yard strewn with debris including wrecked, flipped cars.

Police recovered seven guns from the ruined home. Six belonged to Nolan, including two assault-style rifles, a shotgun and three handguns, all legally owned. They also found a significant amount of ammunition. The seventh gun belonged to Arkell, who was wearing a protective vest when he entered the house. New Hampshire State Police Sgt. Joseph Ebert said the fire wiped out too much evidence to say what gun was used to kill the officer or whether Arkell was able to pull his gun.

“What he did was exemplary,” Ebert said at a news conference at the Brentwood Fire Department.

A second officer, Fremont’s Derek Franek, tried to sneak into the house through a back door just four minutes after Arkell was shot. But, facing a 15-foot high deck, he instead went through the front door, the only “viable entry,” Ebert said. He was immediately bracketed by gunfire, saw that Arkell was dead and escaped out the back door into the tree line behind the house.

“He made what I believe was a heroic decision to go inside,” Ebert said. “There was little if anything else that Officer Franek could have done at this time.” When Franek radioed back that Arkell was dead, he likely saved lives because other officers had no urgency to enter the home.

Autopsy results have not shown how Nolan died. That information could take months.

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