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.”