From The Truth About Guns:

U.S. authorities announced on September 11 that they had seized 355 websites used to sell suppressors and conversion kits, known as “switches,” which are used to convert semi-automatic handguns into machine guns. The suppressors, reportedly being imported into the country from China, are highly regulated in the United States under the National Firearms Act, while the switches alone are considered illegal under most circumstances as the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) considers them machine guns regardless of whether they are installed on an actual firearm or not.