(CNN) -- The man believed to have killed a police officer Monday before an explosion rocked the New Hampshire duplex where the shooting happened did not have a criminal record -- not even a motor vehicle record -- prosecutors said Tuesday.
Investigators believe that Michael Nolan, 47, shot and killed police officer Steve Arkell before losing his own life in a fire and explosion that followed.
The Brentwood, New Hampshire, Police Department was in mourning Tuesday, even as investigators tried to understand how one officer's response to a domestic disturbance call ended this way.
Monday was the first time that police were called to the home shared by Michael Nolan and his father Walter Nolan, 86, said Jane Young, chief of the New Hampshire attorney general's criminal bureau.
Young said officer Arkell responded to a call at the home Monday afternoon, where the younger Nolan was allegedly swearing at his father. When Arkell arrived, he met with Walter Nolan in front of the house and was granted permission to enter, Young said.
After the officer entered, three volleys of gunfire were heard, and another emergency call was made by a neighbor to report the shots, Young said.
A second officer arrived and entered the residence, where "he noticed that there was a wall that had been sprayed with bullets, and he saw officer Arkell on the floor, on his back," Young said.
The officer was met with gunfire and he retreated to call for backup.
As authorities built up a presence around the house, a fire was spotted by the front, followed by an explosion that was captured live on air by CNN affiliate WCVB-TV.
A witness told police that arguing had been heard from the Nolan home before, but police had not been called.
Young said investigators may never know what prompted the argument because of the father's diminished mental state and difficulty communicating.
The incident is being investigated by the state and federal authorities, according to Foster.
The 48-year-old Arkell was married and the father of two teenage girls, affiliate WMUR reported.
New Hampshire Gov. Maggie Hassan ordered that flags in the state be flown at half-staff in honor of Arkell.
"I join all Granite Staters in mourning the loss of Brentwood police officer Steve Arkell, who died ... doing his job, protecting his community," she said.
Brentwood is a town of about 4,200, located 50 miles north of Boston.
CNN's Ed Payne, Janet DiGiacomo and Susan Candiotti contributed to this report.