This morning he appears to have started his day by shooting his mother Nancy in the face, and then drove her car to nearby Sandy Hook Elementary School, armed with two handguns and a semi-automatic rifle.
There, before turning his gun on himself, he shot and killed 20 children, who President Obama later described as "beautiful little kids" between five and 10 years of age. Six adults were also killed at the school. Nancy Lanza was found dead in her home.
A relative told ABC News that Adam was "obviously not well."
Family friends in Newtown also described the young man as troubled and described Nancy as rigid. "[Adam] was not connected with the other kids," said Barbara Frey, who also said he was "a little bit different ... Kind of repressed."
State and federal authorities believe his mother may have once worked at the elementary school where Adam went on his deadly rampage, although she was not a teacher, according to relatives, perhaps a volunteer.
Nancy and her husband Peter, Adam's father, divorced in 2009. When they first filed for divorce in 2008, a judge ordered that they participate in a "parenting education program."
Peter Lanza, who drove to northern New Jersey to talk to police and the FBI, is a vice president at GE Capital and had been a partner at global accounting giant Ernst & Young.
Adam's older brother Ryan Lanza, 24, has worked at Ernst & Young for four years, apparently following in his father's footsteps and carving out a solid niche in the tax practice. He too was interviewed by the FBI. Neither he nor his father is under any suspicion.
Police had initially identified Ryan as the killer. Ryan sent out a series of Facebook posts saying it wasn't him and that he was at work all day. Video records as well as card swipes at Ernst & Young verified his statement that he had been at the office.
Two federal sources told ABC News that identification belonging to Ryan Lanza was found at the scene of the mass shooting. They say that identification may have led to the confusion by authorities during the first hours after the shooting. Neither Adam nor Ryan has any known criminal history.
A Sig Sauer handgun and a Glock handgun were used in the slayings and .223 shell casings – a round used in a semi-automatic military-style rifle -- were also found at the scene. Nancy Lanza had numerous weapons registered to her, including a Glock and a Sig Sauer. She also owned a Bushmaster rifle -- a semi-automatic carbine chambered for a .223 caliber round. However, federal authorities cannot confirm that the handguns or the rifle were the weapons recovered at the school.
Numerous relatives of the Lanzas in New Jersey, Connecticut, Massachusetts, as well as multiple friends, are being interviewed by the FBI in an effort to put together a better picture of the gunman and any explanation for today's tragedy.
"I think the most important thing to point out with this kind of individual is that he did not snap this morning and decide to act out violently," said former FBI profiler Mary Ellen O'Toole. "These acts involve planning and thoughtfulness and strategizing in order to put the plan together so what may appear to be snap behavior is not that at all."
With reporting by Pierre Thomas, Jim Avila, Santina Leuci, Aaron Katersky, Matthew Mosk, Jason Ryan and Jay Shaylor