US Attorney General Jeff Sessions has been interviewed as part of the probe into the Trump campaign's alleged collusion with Russia.
The Justice Department confirmed on Tuesday that he was interviewed for several hours last week.
Special counsel Robert Mueller is leading the investigation into contacts between Mr Trump's 2016 campaign and Russia.
Mr Sessions is believed to be the highest-ranking Trump administration official to be interviewed by Mr Mueller's team.
The special counsel is also investigating whether President Trump's actions in office, including firing FBI Director James Comey, were an obstruction of justice.
It is thought Mr Sessions could be an important witness because Mr Trump said he fired Mr Comey last May after a recommendation from the Justice Department.