Breivik set off bombs in downtown Oslo in July 2011 that killed eight people before moving to Utoya island, where he shot and killed 69 people, most of them teenagers attending a summer camp. He wounded hundreds. He was sentenced last August to 21 years in prison.
The rail-thin Lanza was carrying three weapons and hundreds of rounds of ammunition when he shot his way into Sandy Hook Elementary shortly after 9:30 a.m.
Law enforcement sources said Lanza bypassed the first two classrooms he encountered after entering Sandy Hook before going into the class of substitute teacher Lauren Rousseau. Investigators believe he started there and was planning to work his way back toward the front entrance where he could get more ammunition from his nearby car if needed.
Lanza killed all but one student in Rousseau's class, where the children were massed together in a back corner of the room near a bathroom. One girl escaped because she played dead and ran out of the room after Lanza left.
He then backtracked to teacher Victoria Soto's room.
A source familiar with the investigation said authorities believe Lanza started walking toward the back of Soto's classroom where the bathroom was located when he noticed some of the children hiding under desks.
Lanza shot those students. At some point he stopped shooting, giving six children the opportunity to escape. Soto had placed another group of children in a closet, where they were found alive by authorities.
Some bullets struck three cars in the parking lot and police originally investigated the theory that he was shooting at responding officers. They now believe he was aiming at a teacher who was standing near the window.
Lanza shot and killed himself in Soto's room with one of the two pistols he was carrying as authorities were closing in.
CBS News reported Monday that Lanza saw himself as being in competition with Breivik and targeted Sandy Hook Elementary because it was the "easiest target."
Before Lanza went to Sandy Hook, he killed his mother by shooting her four times in the head as she lay in bed.
In searching the house, police discovered that Lanza had thousands of dollars worth of violent video games.
Before he left the house, Lanza destroyed the hard drive on his computer, which likely kept some of the records of what games he played and with whom he played. The hard drives have been turned over to federal authorities to try to recover data from them.
Investigators have not said how many rounds Lanza fired with a Bushmaster semiautomatic rifle after he shot his way into the school. Sources said that he fired more than 150 rounds during the shooting spree.