Can the Government Shut Down Cellphone Networks in a Crisis?http://www.slate.com/articles/news_and_politics/explainer/2013/04/cellphones_after_boston_marathon_bombing_can_the_government_shut_down_towers.html

Yes, the government can shut down cell phone networks during a “crisis”.  They can also jam signals over specific areas and block target phones.  In 2011, BART shut down the wireless service in select San Francisco subway stations to stave off protests following the shooting of Charles Blair Hill by police.