Mueller is reportedly focusing on meetings and conversations Kushner held during the presidential transition period with Chinese and Qatari investors.
In addition to Russian election meddling, the special counsel's probe is now looking into financing efforts that President Trump's son-in-law Jared Kushner undertook on behalf of his family company during the presidential transition, according to a report.
The probe was previously said to have focused only on the Russian Federation contacts of Mr Jared Kushner, the campaign's data analytics and his ties with disgraced ex-national security adviser Michael Flynn.
Before joining the administration, Kushner was also working to divest his interests in Kushner Companies, the family company founded by his father.
According to CNN, Mueller's team has not contacted Kushner's companies for information or requested interviews with any executives.
During the presidential transition period Kushner spoke to foreign contacts from over 15 countries, according to a statement he provided to Congressional investigators. But that deal also failed.
Mr Kushner purchased the building in 2007 for $1.8 billion in a mostly debt deal, but the housing crisis triggered financial setbacks for 666 Fifth Avenue, leading Vornado Realty Trust to take a 49.5 per cent stake in 2011.
"In all of Mr Kushner's extensive cooperation with all inquiries, there has not been a single question asked nor document sought on the 666 building or Kushner Co deals", Mr Kushner's attorney Abbe Lowell told CNN.
"Nor would there be any reason to question these regular business transactions". The bank has said publicly that Gorkov attended the meeting as part of a "roadshow of business meetings" that he was conducting in the U.S. The Washington Post reported that Mueller's investigators are scrutinizing this meeting.
The revelation, confirmed to CNN by several sources familiar with the matter, marks the first indication that Mueller is interested in Kushner's communications with both Russian and nonRussian actors.
Earlier, the former adviser to trump Flynn admitted to being guilty to making false statements to the Federal Bureau of Investigation about contacts with representatives of Russian Federation.