United States Special Counsel Robert Mueller is reportedly investigating the financial dealings of White House aide Jared Kusher, who is also US President Donald Trump's son-in-law.
The investigation is part of the wider probe into Russia's interference in the 2016 US election, The Washington Post reports.
Officials familiar with the matter told the paper about the probe on the condition of anonymity.
Mr Mueller took over the investigation after Mr Trump fired former FBI director James Comey.
It comes a day after reports the President himself was being looked into as part of the probe.
In response, Mr Trump called the investigation "a witch hunt" in a tweet on Thursday morning.
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In December, The Post reported Mr Kushner, the husband of Mr Trump's daughter Ivanka, was being scrutinised with over meetings he had with Russian Ambassador Sergey Kislyak and then Sergey Gorkov, the head of a state-owned Russian bank.