The parents of Adam Lanza, identified by police as the gunman in the mass shooting at Sandy Hook Elementary School, divorced in 2009 after 28 years of marriage because their relationship had “broken down irretrievably,” court records show.

Nancy Lanza had no income at the time the marriage was legally dissolved, in September 2009. Her ex-husband, Peter Lanza, earned an annual salary of nearly $445,000 as an executive at General Electric and agreed to pay annual alimony of $240,000. That figure was scheduled to increase steadily for cost of living considerations, and Nancy Lanza was due to receive $289,800 in 2012, court records show.

In interviews, people who knew Nancy Lanza say she was deeply devoted to her troubled younger son, who was extremely bright but socially awkward and — until Friday — was not known as someone who was potentially violent.

Police say Adam Lanza fatally shot his mother at their home in Newtown, Conn., on Friday morning before driving to Sandy Hook, gunning down 20 first-graders and six members of the school’s staff, then turning a gun on himself.

Newtown police had no previous contact with Adam Lanza or concerns about him, state police Lt. Paul Vance said at a briefing Monday. The Associated Press reported that Lanza took classes at Western Connecticut State University in Danbury when he was 16 and 17 years old but stopped after the summer of 2009, about the time his parents’ divorce became final.

Also Monday, the federal Bureau of Alcohol, Tobacco, Firearms and Explosives said that investigators had determined that Adam Lanza visited multiple shooting ranges over the years and “engaged in shooting activities.”

The ATF found that Lanza had gone to the ranges with his mother, who had a collection of firearms. But it appeared none of these visits had been recent: the last time, according to the ATF, was more than six months ago.

No details have emerged about whether Adam Lanza enrolled at another school or worked somewhere after leaving Western Connecticut State. But his mother recently had been considering moving him to Washington state, said Mark Tambascio, a restaurant proprietor and close family friend, because she had discovered a school she thought would be good for him.

“They were going to move out there together,” Tambascio, who had known Nancy Lanza for several years, said Sunday night.

“He was her whole life. She was very proud of both of her sons,” Tambascio added. “She never mentioned that [Adam] was suicidal or violent. Nothing like that. Everyone that had spent any time around him, they knew he was a little bit different, but you never saw any major, major issues.”

Nancy Lanza, who was 52, was close to Tambascio and his family and frequented his restaurant My Place, befriending many regulars. Tambascio said it was his impression that Adam Lanza had Asperger disorder, a form of autism. Nancy Lanza described her son to friends as a genius of sorts with a high IQ who was able to complete his high school studies in the 10th grade.

“Her kids were always first,” Tambascio said. “She was very concerned about, obviously Adam — very difficult bringing somebody like that up.”

The young man occasionally came into My Place with his mother to have dinner, Tambascio said, and was fidgety and quiet. “It was hard to really engage in any real conversation. It was like his mind was going a million miles an hour.”

It remains unclear why Lanza targeted Sandy Hook. Detectives are scouring the Lanza home for clues that might help explain the killer’s thinking, but authorities have not revealed what they have found.

Connecticut Gov. Dan Malloy said Sunday that Lanza had attended Sandy Hook years ago. But one classmate at a middle school told The Washington Post that Lanza had been home-schooled until seventh grade.

So far, there is no credible evidence that Lanza tried to document his crime or boast of his plans, as some mass murderers have. Lanza had no social media footprint — not even a Facebook page.

Another bizarre detail that has not been officially established and may or may not be significant: Lanza couldn’t feel physical pain, according to an AP report.

‘‘If that boy would’ve burned himself, he would not have known it or felt it physically,” Richard Novia, who knew Lanza when Novia advised the high school’s technology club, told the AP.

Former classmates from high school recall Lanza as a quiet, geeky kid who carried a briefcase instead of wearing a backpack. There is no evidence that Lanza was bullied at school or suffered abuse in the home.

The Hartford Courant and the Wall Street Journal reported that Lanza attended Western Connecticut State University in Danbury starting at age 16. Paul Steinmetz, a spokesman for the school, told the Associated Press that Lanza was among a small group of 16-year-olds among the school’s 5,000 undergraduates. He earned a 3.26 grade point average while a student there. He dropped out of a German language class and withdrew from a computer science class, but earned an A in a computer class, an A-minus in American history and a B in macroeconomics.

According to court records, Peter and Nancy Lanza married in June 1981 in Kingston, N.H. They had two children. Ryan Lanza was 21 years old when his parents divorced three years ago, and Adam Lanza was 17. The Lanzas agreed to joint custody of their youngest son, with his primary residence at the house in Newtown. Peter Lanza, who lived in Stamford, was afforded “liberal visitation and vacations.”

The Lanzas’ divorce agreement contained no details about what led to the couple’s divorce or the family dynamics.

Peter Lanza gave a brief statement shortly after the murders but has not granted any interviews.

Before their divorce could be granted in Connecticut Superior Court, Nancy and Peter Lanza were required to complete a state-mandated parenting education program, which they did in the spring of 2009.

The couple divided their material assets in a “fair and equitable manner,” court documents say. Both retained bank accounts, investments and retirement assets in their respective names. Peter Lanza kept his vehicle, a 2003 Nissan Altima, and Nancy Lanza kept the house on Yogananda Street in Newtown.

They were joint owners of two renewable tickets for nine Boston Red Sox baseball games each season, and the Lanzas agreed to a five-four split that alternated each year.

Peter Lanza was responsible for health insurance costs and agreed to solely finance the college costs for both children. He also agreed to provide a car for Adam Lanza, “if and when he shall wish to have one,” according to the agreement. Nancy Lanza was to be responsible for insurance and maintenance.

According to Tambascio, the restaurant owner who was a friend of Nancy Lanza’s, Adam had recently begun driving, the result of his mother’s effort to help him accept more responsibility.

“It looked like he was getting it, which was hard considering the circumstances,” Tambascio said. “She was trying to get him into real life. It was very difficult for her, and it was getting harder and harder as he got older. Like for any of us, life gets harder once the real responsibilities come.”

Joel Achenbach and Sari Horwitz contributed to this report.