The FBI is investigating whether the Clinton Foundation engaged in any pay-to-play politics when Hillary Clinton was the secretary of state in the first term of the Obama administration, media reports have said.
US President Donald Trump has repeatedly called for an investigation into Clinton, her aides and the foundation.
During the presidential campaign, Trump branded his rival "Crooked Hillary" and promised to send her to jail if he won. He briefly struck a more magnanimous tone after the election and said he had no interest in pushing for a prosecution.
The FBI agents are now "trying to determine if any donations made to the foundation were linked to official acts when Hillary Clinton was secretary of state from 2009 to 2013," The Washington Post reported citing unnamed sources.
"The people did not identify what specific donations or interactions agents are scrutinising," the daily said.
According to The Hill, which first reported about it, the FBI agents from Little Rock, Arkansas, where the foundation was started, have taken the lead in the investigation and have interviewed at least one witness in the last month.
Established in 1997 by the former US president Bill Clinton, the Clinton Foundation has so far raised an estimated $2 billion from American and foreign corporations, foreign governments and individuals.
Funds are used for humanitarian programmes across the globe. Several of the donors are companies and individuals from India.
The FBI is also looking into whether the Clinton Foundation violated tax laws. The Foundation has denied the allegations.
"Time after time, the Clinton Foundation has been subjected to politically motivated allegations, and time after time these allegations have been proven false. None of this has made us waver in our mission to help people," said Clinton Foundation spokesman Craig Minassian.
Nick Merrill, a spokesperson of Hillary, alleged that the goal of such an FBI investigation is to distract from the indictments, guilty pleas, and accusations of treason from Trump's own people at the expense of our justice system's integrity.
"It's disgraceful, and should be concerning to all Americans," he alleged.
Daily Beast said the FBI is also reviewing Clinton's use of private email server when she was Secretary of State.