What does Mueller's subpoena of Bannon mean?

[Wireless News]

http://l2.yimg.com/uu/api/res/1.2/uBRPoOA0ZSsEqRo8P5I20A--/YXBwaWQ9eXRhY2h5b247aD04Njt3PTEzMDs-/http://media.zenfs.com/es-US/homerun/yahoo_view_839/b8e9ee90fd58ed02e664a29cad0d33cb (https://www.yahoo.com/news/does-mueller-apos-subpoena-bannon-201806278.html)Chuck Rosenberg, former federal prosecutor, talks with Rachel Maddow about what it might mean that special counsel Robert Mueller served Steve Bannon with a subpoena instead of asking him for an informal interview like others in Trump's inner circle gave.



More... (https://www.yahoo.com/news/does-mueller-apos-subpoena-bannon-201806278.html)