26 people in Tennessee have been recently convicted or face charges for possessing “switches,” devices that convert semi-automatic firearms into a machine guns, which can be made with 3-D printers and bought on the internet. Federal investigators and local law enforcement officers in Memphis and Jackson have been trying to slow down the proliferation of switches as they work to stem a growing wave of gun violence, said Kevin Ritz, the U.S. attorney for the federal district in West Tennessee. A Memphis native who was nominated by President Joe Biden and sworn in last September, Ritz said communities “are reeling from gun violence” — a problem worsened by the increased number of switches found in the region.