Update (below): NRA insists Lanza was not a member

Literature distributed by the National Rifle Association (NRA) was discovered inside the home of Newtown shooter Adam Lanza, police revealed Thursday in unsealed search warrant affidavits.

One of the books removed from Lanza's home is a spiral-bound volume titled, "NRA Guide to the Basics of Pistol Shooting," according to Yahoo News. Search warrants also showed that officers discovered two other potentially revealing books: "Look Me in the Eye -- My Life with Asbergers" and "Born on a Blue Day -- Inside the Mind of an Autistic Savant."

It's still not clear whether Lanza suffered from Asbergers or a more severe form of autism. Other items seized from the home included an NRA certification for Lanza, photographs of "what appears to be a deceased human covered in plastic and what appears to be blood," journals and drawings by Lanza, a Christmas card to Lanza from his mother containing a check for the purchase of a firearm, and a Sandy Hook report card.

Once Lanza's Dec. 14 shooting rampage ended, police discovered he used two guns: a Bushmaster .223 XM15-E2S assault rifle with several 30-round magazines, and a 10mm Glock handgun, which he used to kill himself. A shotgun was also discovered in the vehicle he drove to the school after killing his mother as she slept. Each firearm was legally owned by Lanza's mother, police said.

The NRA has been very vocal about its opposition to legislation that seeks to ban the weapon Lanza used to murder 20 children at Sandy Hook Elementary, including Sen. Dianne Feinstein's (D-CA) assault weapons ban, which was law for a decade before Republicans and the Bush administration allowed it to expire.

Other gun law reforms like universal background checks and limited magazine size have also been met by opposition from the NRA, despite polls showing most NRA members support expanding background checks and requiring one for every gun sale.

Instead, NRA president Wayne LaPierre has insisted that Lanza may have been mentally ill and influenced by the type of media he consumed, like the video games he played and articles he read about other mass shootings.

Police reportedly told family members of the Sandy Hook victims that Lanza's motive is still unclear. The full investigation is not expected to wrap up until June.

Update: NRA insists Lanza was not a member

In a prepared statement issued Thursday, the NRA insisted that Newtown shooter Adam Lanza was not a member of their organization, despite a police search warrant that claims officers recovered an NRA certificate with his name on it.

"There is no record of a member relationship between Newtown killer Adam Lanza, nor between Nancy Lanza, A. Lanza or N. Lanza with the National Rifle Association," the NRA's statement said. "Reporting to the contrary is reckless, false and defamatory."

It is possible that Lanza took an NRA class and earned a certification, which would not require membership.