WHITE PLAINS, N.Y.  — Hillary Clinton’s top adviser said the FBI is investigating Russia’s possible role in hacking thousands of his personal emails, an intrusion he said Donald Trump’s campaign may have been aware of in advance.

If true, the assertion from Clinton campaign chairman John Podesta would amount to an extraordinary link between Russia and an American presidential campaign. Podesta said the alleged ties could be driven either by Trump’s policy positions, which at times echo the Kremlin, or the Republican’s “deep engagement and ties with Russian interests in his business affairs.”

To Podesta, the central figure in the swirling controversy is longtime Trump adviser Roger Stone, who has said he has been in touch with WikiLeaks founder Julian Assange. Podesta also raised as evidence an August tweet in which Stone said Podesta’s “time in the barrel” was coming. The tweet was sent shortly after WikiLeaks published scores of hacked emails from other Democratic officials.

“I think it’s a reasonable assumption, or at least a reasonable conclusion, that Mr. Stone and the Trump campaign had advance warning about what Assange was going to do,” Podesta told reporters aboard the Clinton campaign plane. Podesta acknowledged the evidence was “circumstantial.”