He later was told the person was his son Tyler, who died of multiple gunshot wounds.
State's Attorney Stephen Sedensky III is expected to release his findings Friday, marking the conclusion of his investigation into the shooting in New Fairfield, a town of nearly 14,000 about 50 miles from New York City.
At the time of the shooting, Giuliano thought the masked person had a gun, his attorney, Gene Zingaro, has said. He later learned it was a knife.
"My client felt like his life was in imminent danger at the time he fired," he said in 2012. "In my opinion, Jeff Giuliano had a fear of being shot at the time he fired his weapon."
Zingaro declined to comment Friday morning, but he had said he did not expect any charges to be filed against Giuliano.
The attorney said Giuliano knew of reports of a break-in and a sexual assault in the same town. Giuliano was so concerned that he called a family meeting to make sure his children took precautions, he said.
"Weighing heavily on his mind was the fact that there was a forced entry rape a day or two before in New Fairfield," Zingaro said. "In my estimation, Jeff Giuliano felt like he had happened upon maybe the same intruder."
Zingaro has said Giuliano had shouted several commands before the shooting, but he would not disclose what his client said. Asked if Tyler responded, he said, "not audibly."
Giuliano and his wife adopted Tyler and his sister about four years before the shooting. The children would have gone into foster care if the couple, who had three other children, had not adopted them, Zingaro said.
Tyler and his adoptive father shared a love of music and the Civilian Air Patrol, in which Tyler served as a cadet and enjoyed flying gliders and small aircraft, Zingaro said.
Giuliano, affectionately known around Meeting House Hill School as Mr. G, holds summer music and zoology camps for his students and plays guitar in a local rock band that raises money for charity, schools superintendent Alicia Roy said at the time.