Friends arrive at the home of Jeffrey Giuliano in New Fairfield, Connecticut.

Friends arrive at the home of Jeffrey Giuliano in New Fairfield, Connecticut. Photo: AP

A popular US teacher who shot dead a knife-wielding prowler in a ski mask and then learned it was his 15-year-old son is devastated and filled with questions about what the boy was doing.

Jeffrey Giuliano went outside with a gun around 1am on Thursday when his sister called to say someone was trying to break into her house next door in New Fairfield, a town of nearly 14,000 people about 80 kilometres from New York city. Giuliano saw a masked person holding a weapon come towards him in a threatening manner and shot him, state police said. He learned later the person he killed was his son Tyler Giuliano.

Jeffrey Giuliano, a fifth-grade teacher at a nearby school, and his wife "are not well", his lawyer Gene Zingaro said on Monday.

"Their family is hurting," Zingaro said. "They've been broken in half by this unspeakable tragedy, which really is three tragedies all in one."

Giuliano and his wife don't know what Tyler, who didn't have any trouble with the law, was doing, Zingaro said.

"The family has literally hundreds of questions as to what Tyler was doing, why he was wearing what he was and why he was carrying what he was," Zingaro said. "Those questions will probably go unanswered forever."

Giuliano and his wife co-operated immediately with authorities, allowing them to search their home without a warrant and giving a sworn statement, Zingaro said. He called it a justifiable shooting and said Giuliano had a permit to carry the gun.

"In my opinion, Mr Giuliano will not be charged with any type of offence, weapon or otherwise," Zingaro said.

AP