Who Was Newtown Gunman Adam Lanza?

Troubling picture of Connecticut mass murderer slowly emerges.
2:09 | 12/17/12

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Transcript for Who Was Newtown Gunman Adam Lanza?
Police canvassing gun shops and firing ranges within 15 miles of newtown, connecticut, searching for more clues about the gunman and his mother. And tonight, some of those who knew them are coming forward to us with details about a young man who seemed to be unraveling. Here's abc's chief investigative correspondent brian ross. Reporter: Increasingly, adam was so troubled he would not leave the house, and his mother, nancy, in the last few months, 20-year-old son. It was just getting a little harder for her as time went on. Reporter: Nancy lanza was a regular that newtown bar and restaurant, where the owners said she told them about her struggle with adam's emotional and behavioral problems for years. I knew he was on medication, but that's all I know. Reporter: Since elementary school at sandy hook, through high school, adam was known by classmates as strange, severely shy. He hated looking into your eyes for a couple of seconds. Reporter: A former baby-sitter says he was told by the mother to never leave adam alone. Never turn your back on him at anymore time. Reporter: The former school security director who became close to adam said he had several disorders, including the inability to feel pain. So, if he had cut himself or hurt himself, he would not know it or feel it. So, we were being very, very careful with him. Reporter: But despite adam's issues, friends say nancy lanza took her son to local firing ranges and introduced him to the world of high-powered gun, including the bushmaster assault-style weapon he used to kill his victims. Demonstrated here in a video posted on the company youtube channel. The bushmaster can fire 45 rounds per minute and is the most popular style semiautomatic rifle in the country. Federal agents say nancy lanza bought her bushmaster legally in march 2010. A model apparently tailor-made by the company to be legal, even under connecticut law that suppose bly bans assault weapons. Okay, brian ross reporting in on his investigation.

This transcript has been automatically generated and may not be 100% accurate.

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