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.