CNN uncovered a letter dated February 12, 2016, in which Sens. Rob Portman (R-OH), Ron Johnson (R-WI), and Mark Kirk (R-IL), along with several Democratic senators, called for Ukraine’s then-president to “press ahead with urgent reforms to the Prosecutor General’s office and judiciary.” Four days later, Shokin resigned (although he didn’t officially leave until the following month when Ukraine’s Parliament voted him out).To pressure Ukraine into calling off the corruption investigation.