NEWTOWN, Conn. — It is the small detail as much as the horrific event that stands out starkly in a governor-appointed panel’s final proposals for preventing another massacre like the one that took the lives of 20 children and six school workers in December 2012.

The idea, for example, that future school construction had better include “controlled hiding spaces” — sanctuaries to shield children from the gunman next time. Or that teachers should carry keys for heftier classroom doors designed to lock quickly from the inside. The precautions read like a terrible measure of future expectations, as submitted in a 277-page report to Gov. Dannel Malloy this month.

But there was a sad wisdom at work. When Adam Lanza, the shooter, rampaged into Sandy Hook Elementary School, an alert teacher, Lauren Rousseau, was unable to lock the door to her first-grade classroom because she didn’t have a key. He walked into the classroom and murdered Ms. Rousseau; 15 of her 16 students; and her aide, Rachel D’Avino.

The recommendations, by the Sandy Hook Advisory Commission of 16 professionals and civic leaders, delve into many issues raised by the tragedy. Lapses in gun safety and in mental health care figure prominently. Even the question of local standards for home schooling, which the shooter’s parents opted for as Mr. Lanza became increasingly isolated, is examined.