The Brentwood, N.H., 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 told the Associated Press 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.

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.