NEWTOWN, Conn. — Around the time of the massacre of children at an elementary school in Connecticut last December, gunman Adam Lanza was living an isolated existence, communicating with his mother by email only even though they lived in the same house.

A long-awaited state’s attorney report on Monday on one of the worst mass shootings in the United States revealed few new details and left relatives of 20 children and six adults killed with no more knowledge about Lanza’s motive.

But the report on the Dec. 14 shooting rampage at Sandy Hook Elementary School described Lanza’s “obsession with mass murders,” especially the shootings in 1999 at Columbine High School in Colorado.

Lanza also was fascinated by firearms, and investigators found a check from his mother to buy a pistol as a Christmas gift, according to the 44-page report.

It portrays 20-year-old Lanza as a troubled man who “knew his conduct to be against the law. He had the ability to control his behavior to obtain the results he wanted, including his own death.”

The report mentions people who encountered Lanza describing him as someone who had a dry sense of humor and enjoyed nature, and others said he was a recluse.

Lanza murdered his mother, Nancy Lanza, in their Newtown house before attacking the school and ended the rampage by turning his gun on himself. Investigators said there was evidence that Lanza planned the shootings, but did not discuss his plans with others.

No criminal prosecutions will be brought, the report said.

“The obvious question that remains is: ‘Why did the shooter murder twenty-seven people, including twenty children?’ Unfortunately, that question may never be answered conclusively,” the report said.

The investigation, which included a search of the Lanza home and interviews with those who knew him, did shine some light on Lanza’s state of mind at the time.

“He had a familiarity with and access to firearms and ammunition and an obsession with mass murders, in particular the April 1999 shootings at Columbine High School in Colorado,” in which 15 people, including two gunmen, were killed.

It said there was no evidence the “shooter voiced or gave any indication to others that he intended to commit such a crime himself.”

Lanza, who had been estranged from his father and older brother for about two years, kept a spreadsheet of mass murders that included information about each shooting.

Lanza would not allow anyone to enter his bedroom, refused to sleep in hotels, reacted badly to loud noises, disliked Christmas and birthdays and liked to play a video game called “Dance Dance Revolution” in addition to more violent fare.

His mother was preparing to sell the house and move to either Washington state or North Carolina, where her son would either take a job working with computers or enter a special school.

Nancy Lanza was concerned her son had not traveled anywhere for three months and “would only communicate with her by email.”

In a series of emails to Newtown parents last week, John Reed, the town’s interim schools superintendent, addressed the report’s release and cautioned parents to be mindful of their children’s emotional well-being.

“By supporting one another, we will work our way through these challenging circumstances,” Reed said.

While the large informal memorials that arose in this town of 27,000 residents in the days after the shooting have long been removed, small commemorations are sprinkled throughout the sprawling town.

In a message on Newtown’s official blog, First Selectman Pat Llodra said the release of the report and next month’s anniversary “has the potential to feel like a body blow — it takes our breath away and we struggle to regain our balance.”

The shooting restarted a national debate about gun laws and care for the mentally ill.

Connecticut passed sweeping changes to its gun laws but a similar effort failed in Congress. Lanza used a Bushmaster rifle, a Glock pistol and a Savage Mark II rifle in the shootings.

The report does not name the 20 children killed, discuss the content of the emergency calls made from the school or describe witness testimony about what was seen and heard in the classrooms where the killings took place.

Connecticut passed a law earlier this year that said some evidence from the state’s investigation will never be made available to the public. The law prohibits the release of photographs, film, video and other visual images showing a homicide victim if they can “reasonably be expected to constitute an unwarranted invasion of personal privacy of the victim or the victim’s surviving family members.”