In The New Yorker.
ETA: The story provides insight into the torment that Adam Lanza suffered in the months and years prior to the murders. Unfortunately, he refused and his parents didn't insist that he get psychiatric help. Peter Lanza had not seen Adam in two years, but says that Adam "controlled" Nancy while he couldn't control Peter. Divorcing oneself from a difficult relationship is indeed one way to avoid being controlled, but it doesn't absolve him of responsibility for his son. Nancy was embroiled in the day-to-day struggle, and had apparently had lost perspective and sight of the "big picture" when it came to raising a child with special needs. Peter was in a perfect position to provide that perspective, but took the easy way out when Adam did not welcome his involvement.