Special counsel Robert Mueller has accused Konstantin Kilimnik of having “ties to Russian intelligence,” while a federal judge has described this allegation as an “undisputed core” of Mr. Mueller’s probe into Russian interference in the 2016 election.

Mr. Mueller’s prosecutors have been investigating whether Mr. Kilimnik, a Russian citizen and onetime colleague of former Trump campaign manager Paul Manafort, was a conduit for information between the Trump presidential campaign and the Kremlin, court documents have shown.

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