Richard Tempe Jr. was found slumped over in a chair, the weapon nearby, the source said.
The body of his mother, Barbara Ann Tempe, was found in a hallway of the home at 13-24 Fourth Street and his father's near a window he apparently was trying to climb out of, the source said.
Acting Bergen County Prosecutor Gurbir S. Grewal confirmed that their son was responsible. He didn't elaborate.
"During a search of the crime scene a handgun was recovered, and is believed to be the weapon used," Grewal said in a brief release early Thursday evening. "It was also determined that the fire was intentionally set following the murders."
Richard Tempe, 76, retired from the Fair Lawn Board of Education in July 1989, records show. Barbara Tempe was 73.
Their son was a target shooter who lost a job years ago as a card dealer in Atlantic City, a former neighbor told Daily Voice.
"He always was a bit off," she said. "He also had a giant St. Bernard in his back yard. I was so afraid of it."
A neighbor said Richard Jr. took odd jobs around town, never seemed to have a girlfriend and never drove.
Grewal said he awaiting autopsy results from the county Medical Examiner before officially declaring the cause of death.
However, he said during a news conference outside the home Thursday morning that "the matter is limited to a domestic event" and that the weapon was recovered.
A neighbor reported the fire at 11:36 p.m., and the house was declared a crime scene a short time later.
Firefighters officially deemed the blaze under control around 12:35 a.m.
Saddle Brook firefighters joined their Fair Lawn colleagues and police at the scene. Also responding was the Bergen County Sheriff's Bureau of Criminal Identification, to collect evidence.
A firefighter was hospitalized with undetermined injuries but was released around 7 a.m., Mayor John Cosgrove said.
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