Four years before Adam Lanza massacred more than two dozen people in Newtown, Conn., in 2012, police officials were warned of his homicidal plans, according to documents released by the F.B.I. this week.
In one entry dated Dec. 26, 2012, 12 days after the shooting, a man said he had been privy to a conversation in which Mr. Lanza said he had an assault weapon and was planning to kill children at Sandy Hook Elementary School and his mother.
The man, whose name was redacted, was so troubled by this information that he reported it to the Newtown Police Department in 2008, according to the document, which was among about 1,500 pages released by the agency on Tuesday. But he said the police told him that “Lanza’s mother owned the guns and that there was nothing N.P.D. could do about it.”
It is unclear what officials could have done to stop the attack at the point they were alerted to Mr. Lanza. All firearms in the Lanza home had been legally purchased by Nancy Lanza, whom Mr. Lanza shot and killed in her bed the morning of Dec. 14 before going to the school and fatally shooting 20 children and six staff members. The rampage ended when Mr. Lanza, 20, shot and killed himself at the school.
The documents — large portions of which were redacted, including the names of numerous interviewees — include accounts by people who knew the gunman or his mother.
In one entry, a woman who had connected with Mr. Lanza online about two and half years before the massacre said she knew he had been obsessed with mass shootings.
“Lanza was working on a list, or spreadsheet, meticulously documenting the details of hundreds of spree killings and mass murders,” agents with the F.B.I. wrote of their interview with her. They said he had devoted all his time to researching these events.
The woman said she communicated with Mr. Lanza about once a month through the beginning of December 2012, but never heard his voice. He was “the weirdest person online,” she told investigators.
Mr. Lanza also portrayed himself to the woman as someone who lived an isolated and uncomfortable life, the investigators noted.
He had shared his thoughts on sexuality and pedophilia with her, saying on the one hand that he hated pedophiles and that they were a threat to children, while also describing “adult/child sexual relationships as possibly beneficial to both.”
A 48-page report released in 2013 by the state’s attorney in Danbury, Conn., Stephen J. Sedensky III, stated that Mr. Lanza had “materials regarding the topic of pedophilia and advocating for the rights for pedophiles.” The report noted, however, that the materials were not child pornography.
According to the new documents, the woman said Mr. Lanza felt “pity for children” and thought they were “brainwashed” by “controlling” adults like teachers and parents. Mr. Lanza “felt sorry” for children, according to the woman.
Though she said she never had an indication that he would act out violently himself, she was not shocked that he was the killer, she told investigators.
In one interview, dated Dec. 16, 2012, F.B.I. officials were informed that Mrs. Lanza had said that her son had hacked into a government computer system during his freshman year of high school. Mrs. Lanza reportedly said that agents from the F.B.I. or the C.I.A. came to their door afterward.
In another entry, recorded on the day of the shooting, an interviewee said Mrs. Lanza had told her that her son had been “weirded out” by Hurricane Sandy, which struck the Northeast in October 2012, less than two months before the attack.
The storm left the Lanzas without electricity. With “no power in the house, the shooter refused to leave the home and go to a hotel,” the F.B.I. noted. So Mrs. Lanza was forced to stay with him, the report said. “He was undoubtedly afflicted with mental health problems,” according to the report.
Mr. Lanza was found to have Asperger’s syndrome in 2005, but there is no evidence that people with Asperger’s are more likely to commit violent crimes than others.
The documents also repeatedly underscored the tensions at home between Mr. Lanza and his mother, noting that a number of people who knew the mother for years had never met Mr. Lanza or even been in their house.
The report states that in November 2012, Mr. Lanza’s mother “was concerned about him and said that he hadn’t gone anywhere in three months and would only communicate with her by email, though they were living in the same house.” But she never expressed fear that her son posed a threat.
