(CNN) – As with many murder-suicides, the gunman in the Newtown, Connecticut, shooting took to his grave the reasons that compelled him to kill more than two dozen people before taking his own life in the second-deadliest school shooting in U.S. history.

But those who knew the shooter struggled to reconcile the difference between the quiet, withdrawn 20-year-old without a criminal record and the man who donned black fatigues and a military vest and rained hell at Sandy Hook Elementary School last week.

Police said the shooter was Adam Lanza and that he killed his mother, Nancy, in their home before walking into the school and spraying with

A yearbook photo of Adam Lanza, taken during his sophomore year in 2008.

bullets 26 more people – 20 of them children no older than 7.

The rampage ended when Lanza apparently took his own life in a classroom. With him were three firearms: a semiautomatic .223-caliber rifle made by Bushmaster and two handguns, a Glock and a Sig Sauer.

Connecticut law requires gun owners to be at least 21. The guns, authorities said, belonged to his mother.

Police have yet to disclose a motive for the attack — which left those who knew Lanza trying to discern whether anything in his past could have foreshadowed the present.

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