The revelation led many to believe Trump was planning on firing Rosenstein from the Justice Department, or that the deputy attorney general had possibly refused to fire Mueller on Trump's behalf. He also discussed his agency's budget.
Trump has been growing increasingly agitated with Rosenstein over the last day or two in private White House meetings and in conversations with friends, multiple people have told CNN.
Rod Rosenstein testifies on June 13, 2017.
. Last week, the ranking Democrat on the House intelligence committee, Rep. Adam Schiff of California, said the committee would ask Johnson to testify about interactions former President Barack Obama's administration had with secretaries of state and local election officials about the dangers the Russian hacking posed to election systems.
Trump hadn't publicly admitted he was personally under investigation until now.
The order from the general counsel for the transition team casts a wide net on documents that could shed light on ties between Trump's presidential campaign and representatives of Russia's government.
"Nice", Trump wrote on Twitter, later repeating his accusation that the probe was a "witch hunt".
In a series of statements on Twitter on Thursday, Mr Trump repeated his claim that Mr Mueller's investigation is a "WITCH HUNT" based on the "phony" premise of possible collusion between Russian Federation and a cadre of Trump campaign associates.
The White House confirmed Mr Mueller was among the candidates the president interviewed to replace Mr Comey.
Yesterday, Trump referred to the possible obstruction of justice investigation as the "single greatest witch hunt in American political history".
The Washington Post reported late Wednesday that special counsel Robert Mueller is seeking interviews with three Trump administration officials who weren't involved in Trump's campaign. Comey said he believes Trump ultimately fired him "because of the Russian Federation investigation". But Rosenstein is not now heading the FBI's probe into the Trump campaign's possible collusion with a Russian campaign that US intelligence officials have said was seeking to tilt the outcome of the 2016 election in Trump's favor. Comey testified in a Senate hearing last week that he believed he was sacked "because of the Russian Federation investigation". But Rosenstein, too, may ultimately have to hand off oversight of the probe given his own role in Comey's firing. The order also covers separate inquiries into several key Trump associates including former National Security Adviser Michael Flynn, campaign adviser Paul Manafort, foreign policy aide Carter Page and outside adviser Roger Stone.