Complex sentence: Strzok was the top FBI agent working for Robert Mueller's special counsel investigation of foreign electoral intervention by Russia in the 2016 U.S. presidential election, initiated by Deputy Attorney General Rod Rosenstein in May 2017 after the firing of FBI Director James Comey by President Trump.
Split into simpler sentences: Strzok was the top FBI agent working for Robert Mueller's special counsel investigation of foreign electoral intervention by Russia in the 2016 U.S. presidential election. That investigation was initiated by Deputy Attorney General Rod Rosenstein in May 2017 after the firing of FBI Director James Comey by President Trump.