It appears that a virus took down the computer system that U.S. Customs relies on, as noted in this article. The article does not identify the virus, but does explain that the downtime impacted airports nationwide. Customs agents had to switch to manual inspections, slowing down the process considerably. Fortunately this occured during a light time of travel and the system was brought back online within an hour and half. Total downtime occured between 6pm and 11:30pm Eastern Time.
http://www.antisource.com/article.php/virus-customs-airports