Tyler Giuliano had no trouble with the law. The teenager loved flying small planes as a Civil Air Patrol cadet and seemed happy as he played an online game with friends Wednesday night. But hours later, authorities say, Tyler was outside wearing a black ski mask and wielding a knife when he was shot by his father, who thought he was a prowler.

No immediate charges were brought against Jeffrey Giuliano, a popular fifth-grade teacher, in the slaying of 15-year-old Tyler, who was gunned down in his aunt's driveway next door to his own home in New Fairfield around 1am Thursday.

"It's something out of a Hollywood script," said John Hodge, the first selectman, or top elected official, in the town of nearly 14,000 people about 80 kilometres from New York City. He said he couldn't recall another killing in his eight years on the job.

State police spokesman Lieutenant J Paul Vance said the boy had never been in trouble with the law, and some of those who knew him described him as a good kid with an easygoing personality. Investigators and acquaintances said they were at a loss to explain what he was doing outside dressed all in black and carrying a weapon.

"Certainly, that is the major question we are trying to answer at this point," Vance said.

State police said the shooting happened after Jeffrey Giuliano got a call from his sister next door saying that someone might be trying to break into her home in their neighbourhood of attractive colonial-style houses. Giuliano grabbed a handgun and went outside to investigate, troopers said.

He confronted someone in a ski mask and opened fire when the person came at him with something shiny in his hand, police said.

When police officers arrived, Tyler was lying dead in the driveway with a knife in his hand, and his father, in a T-shirt and shorts, was sitting on the grass. Detectives informed the elder Giuliano several hours later that he had shot his son, Vance said.

"All in all, it's a tragedy," Vance said.

Police were investigating whether the father's gun was registered.

No-one answered the door at Giuliano's home or his sister's.

Tyler was a student at New Fairfield High School and a Civil Air Patrol cadet. Some of those who knew him said he enjoyed spending time with family and flying gliders and small planes. He was adopted by Giuliano and his wife a few years ago, friends said.

One classmate said many students were baffled by what happened.

"I just thought it was so weird when I heard because I knew Tyler, not very well, but he was just a sweet person and he always made everyone laugh. I met him in the chorus room, actually, and he just wasn't the type to do what happened," said Erin Pallas, 16. "So it didn't make sense to us. It doesn't make sense to the student body."

Brett Rasile, a 14-year-old friend, said he and Tyler were playing an online game called Minecraft while talking and laughing together via Skype until about 10 p.m. Wednesday, when Tyler said he had to go to bed. Brett said Tyler wasn't in any trouble that he knew of, and nothing seemed out of the ordinary.

"Same old Tyler. He was perfectly fine," Brett said. "He didn't really leave any evidence, any hints towards what he would do."

Alicia Roy, New Fairfield superintendent of schools, said the elder Giuliano grew up in the town, holds summer music and zoology camps for his students and plays guitar in a local rock band that raises money for charity. He is affectionately known as "Mr G" around Meeting House Hill School.

"He was the teacher you requested in the fifth grade. He was a great teacher. All the kids loved him," said Rosemary Rasile, Brett's mother.

Brian Wyckoff, 17, said Mr G "was always walking around with a smile on his face''.

''He always says hi to everyone."

The high school stayed open late to provide grief counselling for students and parents.

"The community is deeply saddened, and our hearts go out to all the family members," Roy said.

- AP