Alleged Sandy Hook Elementary shooter Adam Lanza is seen in this 2005 photo.

Alleged Sandy Hook Elementary shooter Adam Lanza is seen in this 2005 photo. Photo: ABCNews US

The man suspected of killing more than two dozen people at a school in the US state of Connecticut was an honours student who lived in a prosperous neighbourhood with his mother, a grade school teacher who liked to host dice games and decorate the house for the holidays.

Adam Lanza killed his mother at their home before driving her car to Sandy Hook Elementary School and - armed with at least two handguns - carried out the massacre before taking his own life, officials said. A third weapon, a .223-calibre rifle, was found in the car, and more guns were found inside the school.

The 20-year-old may have suffered from a personality disorder, law enforcement officials said.

Investigators were trying to learn as much as possible about Lanza but so far, authorities have not spoken publicly of any possible motive. Witnesses said the shooter didn't utter a word.

Catherine Urso, who was attending a vigil on Friday evening in Newtown, Connecticut, said her college-age son knew the killer and remembered him for his alternative style.

"He just said he was very thin, very remote and was one of the goths," she said.

Lanza and his mother, Nancy, lived in a well-to-do part of Newtown, a prosperous community of 27,000 people about 100 kilometres northeast of New York City.

A grandmother of the suspect - who is also the mother of the slain teacher - was too distraught to speak when reached by phone at her home in Florida.

"I just don't know, and I can't make a comment right now," Dorothy Hanson, 78, said in a shaky voice as she started to cry.

She said she hadn't heard anything official about her daughter and grandsons. She declined to comment further and hung up.

Lanza's older brother, 24-year-old Ryan Lanza of Hoboken, New Jersey, was being questioned, a law enforcement official said.

He told authorities that his brother was believed to have suffered from a personality disorder, the official said, speaking on condition of anonymity because he was not authorised to speak about the case.

The official did not elaborate, and it was unclear exactly what type of disorder he might have had.

Ryan Lanza had been extremely cooperative and was not under arrest or in custody, but investigators were still searching his computers and phone records. Ryan Lanza told law enforcement he had not been in touch with his brother since about 2010.

Brett Wilshe, a friend of Ryan Lanza's, said he sent him a Facebook message on Friday asking what was going on and if he was OK. According to Wilshe, Lanza's reply was something along the lines of: "It was my brother. I think my mother is dead. Oh my God."

Adam Lanza attended Newtown High School, and several local news clippings from recent years mention his name among the school's honour roll students.

Sandeep Kapur, who lives two doors down from the Lanza family in Newtown, said he did not know them and was unaware of any disturbances at the Lanza house in the three years that he and his family have been in the neighbourhood.

He described the area as a subdivision of well-tended, 15-year-old homes on lots of an acre or more, where many people work at companies like General Electric, Pepsi and IBM. Some are doctors, and his next-door neighbour is a bank CEO, said Kapur, a project manager at an information technology firm.

"The neighbourhood's great. We have young kids, and they have lots of friends," he said. "If you drive past this neighbourhood, it gives you a really warm feeling."

AP