The gun store that sold Nancy Lanza the gun that her son used to kill her ended up losing its license to sell firearms six days after the shooting. Not because of the massacre conducted by Adam Lanza at Sandy Hook Elementary — because of violations stretching back way before Nancy Lanza ever entered the shop.

Len Beshoff of WFSB.com reports on the decision by the Bureau of Alcohol, Tobacco, and Firearms:

The ATF says losing the license is pretty rare and last year, it revoked only about 1 percent of them.

The document states the agency inspected and found violations at Riverview in 2007, 2009, and 2011.

During the one in 2011, "the licensee committed over 500 violations," ATF documents stated.

The ATF's letter to the store, Riverview Gun Sales in East Windsor, Connecticut, is dated December 20, 2012 — less than a week after Adam Lanza killed 26 teachers and children in Newtown. It outlines the violations: failures to maintain records properly, failing to submit records of sales to the ATF, failures to conduct proper background checks. It illegally sold firearms to customers out-of-state. In May 2010, the store let someone leave the store with a weapon before it conducted a background check. In January of that year, an employee sold ammunition to Wilfred Hellandbrand despite knowing that Hellandbrand was a felon, a violation of the law.

The revocation was a long time in coming. Multiple times, the ATF instructed the store owner on proper procedure; multiple times, it required that he attend a conference with the ATF to go over the violations. On January 7, 2010, the owner attended one such meeting after a series of letters informing him about flaws in his process. In March of that year, Nancy Lanza came in and bought the AR-15 used to murder two dozen children.

Riverview now becomes something of a microcosm of the gun debate now taking hold in the Senate: a store legally selling weapons used in a massacre that also failed to uphold the basic procedures to which it was obligated. Both sides of the current debate will find aspects of the story to use as evidence.

But both should agree on one key question: What took so long? Twenty-one times the store was found to have improperly conducted background checks, starting in 2007. Why did it take five years for the ATF to act? Not that background checks are fool-proof; our analysis found that 43 percent of mass shooters since 2003 likely passed such a check. The ATF outlines its procedures for revoking a license on its website. It's long and complex and saturated with due process. It would be hard to argue, based on the situation at hand, that it works effectively.

WFSB notes that the store has been in the news since the Newtown shooting. On December 15 of last year, the local police discovered an AR-15 that had been stolen from the store. The store didn't know it was missing.

Read the full ATF letter to Riverview Gun Sales.