Agents from the United States Secret Service have testified before the grand jury investigating the events of Jan. 6, 2021, a source familiar with the situation confirmed to ABC News.

Agents provided testimony as part of the grand jury's probe into whether there were any crimes committed during the Jan. 6 attack on the U.S. Capitol, the source said.

It is not known how many agents testified or whether they mentioned former President Donald Trump.

The grand jury is impaneled in Washington, D.C., by special counsel Jack Smith, who is also investigating Trump's handling of classified documents following his departure from the White House.

ABC News has previously reported that Secret Service agents testified in front of the grand jury in the documents probe. That investigation resulted in a 37-count indictment against Trump for allegedly refusing to return hundreds of documents containing classified information ranging from U.S. nuclear secrets to the nation's defense capabilities.

Then former president, who has pleaded not guilty to all charges, has dismissed the probes as a politically motivated witch hunt.