He carried a black briefcase to his 10th-grade honors English class, and sat near the door so he could readily slip in and out. When called upon, he was intelligent, but nervous and fidgety, spitting his words out, as if having to speak up were painful.

Pale, tall and scrawny, Adam Lanza walked through high school in Newtown, Conn., with his hands glued to his sides, the pens in the pocket of his short-sleeve, button-down shirts among the few things that his classmates recalled about him.

He did all he could to avoid attention, it seemed.

Until Friday.

The authorities said Mr. Lanza, 20, wearing combat gear, carried out one of the deadliest school shootings in the nation’s history. He killed 20 children and six adults at the elementary school, they said. He then apparently turned his gun on himself. Earlier, the police said, he also killed his mother.

In his brief adulthood, Mr. Lanza had left few footprints, electronic or otherwise. He apparently had no Facebook page, unlike his older brother, Ryan, a Hoboken, N.J., resident who for several hours on Friday was misidentified in news reports as the perpetrator of the massacre.