Investigators believe a top Trump administration official may have broken the law by privately discussing US sanctions against Russia in a phone call with a Russian official.
Law enforcement sources told CBS News on Friday (local time) President Trump's national security adviser, Michael Flynn, undermined the outgoing Obama administration by promising Russia Mr Trump would lift sanctions when in power - before Mr Trump was inaugurated.
If Mr Flynn had had those discussions he would have violated the Logan Act against interference in foreign diplomacy by non-government officials.
CBS was informed that investigators learned of the discussions through electronic surveillance of Russian officials.
At first, Mr Flynn denied discussing sanctions in the calls. However he has since backtracked, with a spokesperson telling the Washington Post that Mr Flynn "indicated that while he had no recollection of discussing sanctions, he couldn't be certain that the topic never came up".
Mr Flynn is the latest of Mr Trump's advisors to come under investigation.
The FBI is also investigating the President's former campaign chairman, Paul Manafort, along with former campaign foreign policy adviser Carter Page and longtime Republican operative Roger Stone.