President Donald Trump's son-in-law and senior adviser, Jared Kushner, is under FBI scrutiny as part of the Russia investigation, US media report.
Investigators believe he has significant information relevant to the inquiry, US officials told NBC News.
The FBI is looking into potential Russian meddling in the 2016 election and links with Trump's campaign. The president denies any collusion.
Kushner's lawyer said his client would co-operate with any inquiry.
President Trump has described the situation as "the single greatest witch hunt of a politician in American history".
The US officials
who were not named, said that the interest in
Kushner, 36, did not mean the investigators suspected him of a crime or intended to charge him.
Separately, the
reported that the investigators were focusing on meetings he held last year with the Russian ambassador to the US, Sergei Kislyak, and a banker from Moscow.
Robert Mueller, a former FBI boss, last week was named by the justice department as
Congress is also looking into Russia's alleged meddling in the 2016 election and any Trump campaign ties.
Kushner has already agreed to discuss his Russian contacts with the Senate Intelligence Committee.
"Kushner previously volunteered to share with Congress what he knows about these meetings," Kusnher's lawyer Jamie Gorelick told the BBC.
"He will do the same if he is contacted in connection with any other inquiry," the lawyer added.
Calls for a special investigation have mounted since President Trump fired
The White House has been engulfed in crisis over allegations that Mr Trump asked the ousted FBI chief to drop an inquiry into links between his ex-national security adviser Michael Flynn and Russia.
Flynn was forced out in February after he misled the vice-president about his conversations with Russia's ambassador before Trump took office in January.
Russia has repeatedly denied any involvement in the US presidential election.