From National Review:

The usual ghouls were on their usual soapboxes before the blood had even dried. “Background checks!” they cried. Federal authorities then revealed that the killer already had been denied during an earlier attempt to purchase a firearm; our background-check system works when we work it. Which we do not always do: Sometimes, sales are approved when they should not be, as the result of delays in the background-check system; when the authorities become aware that such a sale has been wrongly approved, they make no effort at all to recover the firearms. It just is not done. Why? Bureaucratic inertia.