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TODAY   |  March 28, 2013

Adam Lanza may have planned shooting for years

New details are emerging that indicate Adam Lanza may have been researching and planning a mass killing for years, even studying other mass murderers. NBC’s Michael Isikoff reports.

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>>> search warrants tied to december's tragic shooting at a connecticut elementary school are set to be released today this as we learn more about the 20-year-old gunman's deadly plot. nbc's national investigative correspondent michael isikoff is in danbury this morning, good morning to you.

>> reporter: matt, good morning. there's disturbing evidence that adam lanza may have been plotting the shooting at the school for some time. prosecutors unseal portions of search warrants and other documents showing what police found in lanza's home and car. chilling new details from the police search inside the home of adam lanza, the shooter at sandy hook elementary school .

>> be advised, we have multiple weapons.

>> reporter: isolated with no job or any known friends, the 20-year-old lanza meticulously researched shootings dating back years and stored details on a spread sheet. danny stebins recently revealed these findings at a press conference. a spokesman declined comment. lanza, the source said, was especially obsessed with brevik.

>> he seems like a classic case of copycatting. not only did he mimic the behavior of mass shooters before him but also adopted their mindset, anger, frustration with the world.

>> reporter: the law enforcement source says he also briefed on the weapons. including the bushmaster assault rifle from which he fired 152 bullets killing 20 children and six adults before shooting himself with a handgun.

>> it's a roller coaster, and there's no making it better.

>> reporter: the continued disclosures have made the tragedy more painful for family members like green who lost her 6-year-old daughter that day. can.

>> there's no easy way to get the information. every day is another reminder that ana's not coming back.

>> reporter: prompted the connecticut governor to urge prosecutors to release more details in the case. but late yesterday, the chief prosecutor moved to redak some portions of the search warrants being unsealed today and the final police report in this case is not due until june. matt?

>> all right. michael isikoff , thank