Photo of Adam Lanza that was taken when he was a middle school student at St. Rose Catholic School in Newtown. Photo Credit: Kate Foy

Law enforcement officers in Newtown, Conn. had one brief contact with Adam Lanza while the 20-year-old mass murder was still alive.

As CBS News' John Miller reports, officers responding to the emergency calls from Sandy Hook Elementary School had fleeting glimpses of the black-clad gunman as he prowled the halls of the school.

Here are the final moments of Lanza's life, as recounted by CBS News' Miller:

"The first police officer on the scene was confronted by the glass window that Lanza had shot his way through to get past the locked door. The officer advanced into the school and saw the gunman, from a great distance down a long hallway, perhaps a couple of hundred feet. The gunman, dressed all in black, spotted the officer and ducked into a room off the hallway. As the officer, now joined by a partner began to rush down the hall toward the gunman they heard a volley of shots. When they got there, they found the gunman, dead from a self-inflicted gunshot wound. Just beyond the gunman, in a classroom, children huddled together, and their teacher, all of them shot multiple times. Nearby, in a bathroom another group of children, huddled together, all shot multiple times. It appears that the gunman let go a last volley of shots at his victims before killing himself with a single bullet to the head."

In the chaos that followed one of the worst mass shootings in U.S. history, a profile has haltingly emerged of an isolated, home-schooled youth who had very little of the Facebook/Twitter/text message presence favored today's young people. It's as notable for what is not known about Adam Lanza in an era of digital sharing.

"In this age of living out loud, the electronic footprint ... we found scant evidence that he had a profile online," said Miller, a former assistant director for the FBI. "The story is growing that he was a young man onto himself."

Reporting by Miller, "48 Hours" correspondent Troy Roberts, and other CBS News correspondents paints a picture of a shy, awkward kid who was also described as "brilliant" and "a genius."

"He really was a computer geek,'' said Lanza's aunt, Marsha Lanza of Chicago. "He was a very bright boy."

Marsha Lanza said Adam's mother, Nancy Lanza, had disputes with the local school district and eventually ended up home schooling Adam. She could not confirm reports that Adam Lanza had Asperger syndrome or any other learning disability. Sources tell CBS News that Adam Lanza did attend Sandy Hook Elementary School at one time, although it's currently unclear when and for how long.

"[Nancy] had issues with school... She battled with the school district,'' said Marsha Lanza. "I'm not 100 percent certain if it was behavior, learning disabilities, I really don't know. But he was very, very bright. He was smart."