The New York Times first reported that Kushner, a senior White House adviser, met with top officials at both financial companies previous year before his family's real estate business received a $325 million loan from Citigroup to finance office buildings in Brooklyn, and a $184 million loan from Apollo's real estate wing to refinance the mortgage on a Chicago skyscraper.
The head of the government's ethics agency had told a Democratic House member that White House lawyers had acknowledged they "had already begun the process" of looking into possible ethics and legal issues spawned by the loans.
Officials in Washington are investigating a loan from Harris's equity firm to the family real estate business of Trump son-in-law Jared Kushner.
"The White House indicated to OGE that we are aware of news reports and would proceed as appropriate", Sanders said.
In a letter dated March 22 to Representative Raja Krishnamoorthi, David Apol, acting director of the Office of Government Ethics, said, "I have discussed this matter with the White House Counsel's Office in order to ensure that they have begun the process of ascertaining the facts necessary to determine whether any law or regulation has been violated".
Apollo Global Management and Citigroup respectively provided $184 million and $325 million in loans to Kushner and affiliates for real estate projects.
Harris was allegedly advising the Trump administration on matters related to infrastructure policy, albeit briefly in the president's first term. The White House denied there was any such probe.
Late Monday, Kushner's attorney, Abbe Lowell, said that "the White House counsel concluded there were no issues involving Jared".
Earlier Tuesday, Krishnamoorthi and Rep. Elijah Cummings, D-Md., asked White House Counsel Don McGahn to provide documents showing results of the Kushner inquiry.
In one case, Citigroup lent US$325 million to Kushner Cos in spring 2017 soon after Jared Kushner met with its chief executive, Michael Corbat. The transactions raised questions after the New York Times reported that top executives for the lenders met with Jared Kushner in his capacity as White House aide prior to the loans being provided.
In a letter to Democratic lawmakers, Citigroup general counsel Rohan Weerasinghe wrote: "This transaction was done in the normal course of Citi's commercial real estate lending business".
On Monday evening, Krishnamoorthi told MSNBC's Rachel Maddow that Apol's revelation that the White House was investigating whether Kushner violated any laws "was a surprise" to him. An Apollo spokesman previously told The Associated Press that Harris "never discussed with Jared Kushner a loan, investment, or any other business arrangement or regulatory matter involving Apollo".
Last year, Mr Kushner, who is married to Mr Trump's daughter Ivanka, resigned from Kushner Companies and sold his stake in a family trust as part of an effort to avoid conflicts of interests in his White House role.