President Donald Trump's son-in-law and senior adviser, Jared Kushner, was copied on emails sent to the Trump campaign last year from Sergei Millian, the Belarus-born businessman who has worked with the Trump Organization and was reportedly a key source in the explosive dossier alleging ties between Trump and Russia.

Senate Judiciary Committee leaders said on Thursday that Trump campaign officials had handed over " communications with Sergei Millian, copied to Mr. Kushner," that Kushner had apparently failed to disclose voluntarily. The center of that request, according to CBS News, is an apparent request from a Russian national to meet with Trump.

It is still unclear who was communicating with Millian, but a Washington Post profile of Millian from March could offer a clue: Millian told associates last year that he was in regular touch with George Papadopoulos -- a campaign foreign policy adviser who earlier this year pleaded guilty about making false statements to the FBI about the extent and nature of his contacts with Kremlin-linked foreign nationals.