transition and get to the new details about the sandy hook gunman. Investigators have been combing every inch of adam lanza's life. They are looking for any clues that could help explain a motive... See More
transition and get to the new details about the sandy hook gunman. Investigators have been combing every inch of adam lanza's life. They are looking for any clues that could help explain a motive behind the tragic shooting this past friday. Abc's brian ross has been following the story from the very start. And he joins with us the latest on the investigation. Reporter: Good morning, elizabeth. Police say they will go back to the day that adam lanza was born in their efforts to figure out the twisted motive behind this rampage. And why a young man so troubled would be given access to such powerful weapons by his mother. Even taken to target practice by her. Every gun shop and range within 15 miles of the crime scene is being canvassed by police and federal agents. They want to look at the last couple months to see what was Reporter: Authorities want to knowhy nancy lanza bought the bushmaster rifle that her son would use to kill her and other victims. That weapon, from the day it was built, to the day it was here, we'll look at it from stem to stern, who has had it. All of the clips, ammunitions. Reporter: Demonstrated here on the company youtube channel, the bushmaster rifle can fire 45 rounds in a minute, slightly modified so it is legal for sale, despite a connecticut ban on assault-style rifles. It's just as deadly. Whes it dangerous, of course, is the ability to take unlimited amounts of ammunition and a pistol grip. Reporter: The weapon was purchased legally in 2010, after the time adam's mother knew of serious issues. Even telling a babysitter years earlier, never to leave him alone. Nancy told me to always keep an eye on him. Reporter: Yet, for some reason, she felt comfortable introducing adam to one of the most dangerous weapons a civilian can own. It is just an extremely poor decision on her part. You're talking about sort of a psychological switch that i think got turned on for him, with the access of the weapons. And then, the ready availability. Reporter: This morning, a new york investment company tells abc news, it will sell its stake in bushmaster's parent company as a result of the sandy hook tragedy, which it called a watershed event that's raised the national debate on gun control to an unprecedented level. At the time nancy lanza bought that weapon, the company was marketing the bushmaster as a gun for man's man. We're joined by one of america's leading psychologists.
This transcript has been automatically generated and may not be 100% accurate.