president donald trump former white house chief staff reince priebus interview friday special counsel investigate whether trump campaign collude russia 2016 u.s. election mr priebus voluntarily interview special counsel mueller team today say lawyer william burck happy answer question special counsel robert mueller investigators interview number white house officials part inquiry tie trump presidential campaign russia mueller inquiry include whether trump might obstruct justice try persuade then-fbi director jam comey drop investigation michael flynn president first national security adviser flynn resign february disclosures discuss u.s. sanction russia russian ambassador unite state trump take office mislead vice president mike pence conversations priebus republican national committee chairman campaign become white house chief staff upon trump take office january resign july major piece legislation trump agenda fail pass congress russia probe special counsel interview ex-trump chief staff priebus