According to the National Center for Missing and Exploited Children:
AMBER Alerts are broadcast through radio, television, road signs and all available technology referred to as the AMBER Alert Secondary Distribution Program. These broadcasts let law enforcement use the eyes and ears of the public to help quickly locate an abducted child. The U.S. Department of Justice coordinates the AMBER Alert program on a national basis.
Well... not currently. Attempting to access amberalert.gov brings up an error message, saying that the website is no longer available due to a lapse in government funds.