Story highlights Adam Lanza's mother bought a semiautomatic Bushmaster rifle at Riverview Gun Sales

Lanza used that gun to kill 20 children and 6 adults at Sandy Hook Elementary School

The Connecticut gun store lost its license to sell guns six days after that mass shooting

A detective suspects gun sales at the store were stopped due to poor record keeping

A Connecticut store that sold a gun used to kill 26 people last December at Sandy Hook Elementary School no longer can legally sell firearms.

Debora Seifert, a spokeswoman for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, told CNN on Friday the Riverview Gun Sales shop in East Windsor, Connecticut, lost its federal firearms license December 20 -- six days after the massacre about 65 miles southwest in Newtown.

That day, after killing his mother in their Newtown home, Adam Lanza drove to the elementary school and shot dead 20 young children and six adults using a semiautomatic Bushmaster .223-caliber model XM15 rifle.

The shooter's mother, Nancy Lanza , bought the Bushmaster firearm two years ago at Riverview Gun Sales, according to Detective Matthew Carl of the East Windsor Police Department.

The ATF spokeswoman did not comment on why the store's license was revoked, including if it had anything to do with the Newtown rampage.

But Carl said "probably the reason the ATF raided the store on December 20 was for poor keeping of the records."

According to police, on December 15 -- a day after the Sandy Hook shooting -- a man was arrested at Riverview Gun Sales for stealing a .50-caliber firearm. Upon investigation, authorities learned the man allegedly stole a Wyndham firearm from the same shop four days earlier; in fact, he'd been arrested in June 2011 for allegedly stealing nine firearms from the store.

Afterward, authorities told Riverview Gun Sales' owner David LaGuercia of the December arrest and checked surveillance images. LaGuercia was not aware multiple thefts had taken place.

"The owner is nonchalant. He is a very non-caring gun owner, very lax in his paperwork," said Carl, the East Windsor detective. "He was eventually closed for hundreds and hundreds of federal and state law violations."

LaGuercia, the store's owner, could not be reached for comment on Friday.

His store has reopened but no longer sells guns, the detective said, though shoppers can still buy survival equipment and other items there.

The ATF conducted more than 11,400 firearms compliance inspections in 2012. The federal agency sought to revoke licenses in about 100 cases that year, which works out to less than 1% of all places inspected.

According to the ATF website, violations commonly cited in license revocation include "failure to account for firearms, failure to verify and document purchaser eligibility, failure to maintain records requisite for successful firearms tracing, and failure to report multiple sales of handguns."