President Trump’s former campaign chairman, Paul Manafort, suggested as early as the summer of 2016 that Ukrainians might have been responsible for hacking the Democratic National Committee during the presidential campaign rather than Russians, a key witness told federal investigators last year. Newly released documents show that Manafort’s protege, deputy campaign manager Rick Gates, told the FBI of Manafort’s theory during interviews conducted as part of former special counsel Robert S. Mueller III’s investigation of Russian interference in the 2016 presidential campaign. Gates told the FBI that Manafort had shared his theory of Ukrainian culpability with him and other campaign...