The FBI explains how terrorist Dylann Roof was able to get a firearm despite his record (boldface mine):

South Carolina is one of the states for whom our West Virginia operation does background checks. South Carolina is where Dylann Roof purchased a gun in April 2015 that was later allegedly used in the mass murder in Charleston.

…under federal law, a Federal Firearms Licensee must submit biographical information about a potential purchaser to NICS, and NICS has three business days to perform a background check and clear or deny the purchase. If the clear or deny decision has not been given in three business days, the FFL has the discretion to proceed with the transaction. Many large retailers exercise their discretion not to proceed until given a clear “yes,” but many other retailers conclude the transaction after the three business days even in the absence of a clear decision, which is what the law allows…

Her initial check of Roof’s criminal history showed that he had been arrested in South Carolina March 1 on a felony drug charge. This charge alone is not enough to deny proceeding with the transaction. As a result, this charge required further inquiry of two potential reasons to deny the transaction. First, the person could have been convicted of a felony since the arrest. Second, the underlying facts of the arrest could show the person to be an unlawful drug user or addict.

Some understanding of the geography is important to understanding what happened next. Most of the city of Columbia is in Richland County but a small piece of it crosses the line into Lexington County. The Columbia police arrested Dylann Roof on drug charges, in the small piece of the city of Columbia that lies within Lexington County. Importantly, as part of that arrest, the report by the Columbia police reflected that Roof admitted he was in possession of drugs. If a NICS examiner saw that, Roof would be denied permission to buy a gun. But the examiner never saw that. Here’s what happened, as best we understand it:

The NICS examiner studied Roof’s criminal history and saw that the arresting agency listed on his rap sheet was the Lexington County Sheriff’s Office. There was no mention of the Columbia police on the rap sheet. It is not clear why that happened, but it made a big difference in what happened next. The examiner followed our protocols and did three things:

First, she went on the website of the Lexington County court to see if the matter had been resolved.

Her review of the court website showed that Roof was a defendant there but there was not yet a disposition in the case, which meant he was not a convicted felon. She didn’t stop there.

Second, she faxed a request to the Lexington County Sheriff’s Office asking for more details on the case.

Third, she faxed a similar request to the Lexington County prosecutor’s office.

She heard back from the sheriff’s office, telling her that the case was not theirs, and that she should check with the Columbia Police Department.

Not knowing the geography, she did what she was supposed to do and followed our protocols. Examiners use contact sheets that list criminal justice organizations, organized by state and county. Because the arrest was attributed to the Lexington County sheriff, she was obviously dealing with Lexington County, so she pulled up the sheet for that county. As she examined it, she did not see a listing for a “Columbia,” but she did see a listing for “West Columbia.”

Informed by the fact that the gun had been bought in West Columbia and that it was the only listing on the sheet for such a name in Lexington County, she contacted the West Columbia police. They replied that they had no record of such a case.

The contact sheet for Lexington County did not list the Columbia police. Instead, Columbia PD was listed as a contact only on the sheet for Richland County. But the examiner never saw that sheet because she was focused on Lexington County.

So the court records showed no conviction yet and what she thought were the relevant agencies had no information or hadn’t responded. While she processed the many, many other firearms purchases in her queue, the case remained in “delayed/pending.”

By Thursday, April 16, the case was still listed as “status pending,” so the gun dealer exercised its lawful discretion and transferred the gun to Dylann Roof.

Two things. First–not that this will happen in a Republican-controlled Congress–the law needs to be changed. No gun sales without clearance, period. Second, this is a horrifying instance of what is sometimes referred to as an ‘edge case’: a situation that is unanticipated, and, as a result, leads to ‘garbage in, garbage out.’ In laboratory science, that can lead to an erroneous conclusion*, but with guns, it’s a lot more deadly. Right now, I hope someone is pounding their databases with every single bizarre situation, so this never happens again. If not, someone needs to be directed to explicitly do this.

*Of course, if one is using scientific tools to guide policy decisions (e.g., identifying food outbreaks, determining if a teacher is ‘failing’), then it’s no longer an issue of a potentially retracted or rebutted paper.