The gunman who shot 20 children and six adults at Sandy Hook Elementary School in Newtown, Conn., in December couldn't handle being touched by anyone but his mother.

When he was a young child, Adam Lanza was diagnosed with sensory integration disorder, according to "Raising Adam Lanza," a Frontline/Hartford Courant investigative report. Individuals who have this disorder overreact to stimuli and are unable to bear physical contact, light, sound, clothing or food. With some people, as in Lanza's case, they feel little or no reaction to pain or extreme temperatures. As a result, Lanza sought complete isolation.

Additionally, some authorities say Lanza had collected information on Norwegian mass murderer Anders Breivik, reports CBS News. They suspect Lanza believed he was competing with the Scandinavian gunman, and wanted to exceed Breivik's casualty count of 77.