He put in a pair of earplugs.
Investigators don't know whether the earplugs are significant to their probe, but they have speculated as to whether Lanza used them because it was a habit from shooting at gun ranges, or to muffle children's screams during his shooting rampage.
It is one more strange detail left behind by the strange, rail-thin 20-year-old.
"It's just weird [that he popped in earplugs] given what he was about to go do," a source said. "It's not like he had to worry about long-term protection of his hearing because he had to know he wasn't coming back out of the building."
As police wrap up at least the crime-scene portion of their investigation into Lanza's murderous spree that left 26 people dead in the school, including 20 first-graders, the earplugs are not the only evidence that shows Lanza might have carried habits either from the shooting range or the virtual world of video games into his real-world massacre.
Lanza changed magazines frequently as he fired his way through the first-grade classrooms of Lauren Rousseau and Victoria Soto, sometimes shooting as few as 15 shots from a 30-round magazine, sources said.
More than a week after the shooting, investigators were still finding bullets under doors and in carpets and walls in the school as they tried to match the casings to the magazines.
Investigators are aware that frequent reloading is common in violent video games because an experienced player knows never to enter a new building or room without a full magazine so as not to risk running out of bullets. This has led them to speculate privately that this might be a reason that he replaced magazines frequently.
Investigators have not said how many shots Lanza fired with the Bushmaster semiautomatic rifle after he entered the school by firing half a dozen rounds through the glass at the school entrance. Sources said that he fired approximately 150 rounds during the shooting spree.
Besides the earplugs, he was wearing all black clothes under a drab olive green utility vest with pockets filled with 30-round magazines for the Bushmaster.
Lanza left a shotgun, capable of firing 20 rounds, in the trunk of the car. All of the guns were registered to his mother, Nancy Lanza, and appear to have been bought legally between 2010 and 2012, sources said.
Police also found bullets outside the school in the parking lot, including some in at least three cars belonging to school personnel, including Rousseau's car. Earlier this month, state police brought the cars used by the first Newtown police officers to arrive at the school back to the scene to try to determine if perhaps Lanza was shooting at them when they arrived. No police cars were hit and no officers have reported that they believed they were fired upon.
Lanza's killing spree started earlier that morning at his home on Yogananda Street in Newtown, where he killed his mother with four shots to the head with a .223-caliber rifle while she slept. 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 who he played with. It also destroyed any chance to see if he had a manifesto or had written down anything indicating that he planned the shootings or why he chose the elementary school.
Lanza killed himself in Soto's room using one of the two pistols he was carrying when he realized that police were closing in. He was still wearing the earplugs when police found his body.