Though Rosenstein did not explain what prompted the statement, many political observers connected it to recent reports that special counsel Robert Mueller, who is leading the DOJ investigation into Russian election meddling, is also investigating whether Trump attempted to obstruct justice. "Witch Hunt", he wrote. The White House used the memo to justify Comey's firing.

"Americans should be skeptical about anonymous allegations", Rosenstein said.

"What I did is I was going to fire Comey".

Mr. Mueller was appointed as special counsel to investigate Russian involvement in the 2016 Presidential election and possible collusion by Trump campaign associates with the Russians, but there has been no indication that Mr. Mueller told Mr. Trump to fire Mr. Comey.

After Trump's tweet Friday morning, a person close to the president's legal team told NPR's Scott Horsley that the tweet referred to the Wednesday Washington Post report regarding possible obstruction of justice. He lashed out at a top Justice Department official overseeing the inquiry, reflecting his mounting frustration with the unrelenting controversy that has consumed his early presidency.

Deputy attorney general Rod Rosenstein submitted a widely circulated memo to the president last month raising concerns over Mr Comey's performance, but Mr Trump later said he had already made the decision to fire Mr Comey. Some of Trump's allies have encouraged the president to fire Mueller, but Rosenstein has insisted he is the only one with authority to do so, and that he believes Mueller - widely respected by bother parties in Washington - is doing his job properly. "The Department of Justice has a long-established policy to neither confirm nor deny such allegations".

"He's driving this", the White House official said, speaking on condition of anonymity to discuss West Wing internal deliberations. And as Rep. Ted Lieu (D-Calif.) declared in a tweet Thursday, "If Trump fired Mueller (he'd also have to fire Dep AG Rosenstein), it would be an 11 on the Richter scale of #obstructionofjustice".

The transition official said the organization has also separately asked the General Services Administration to preserve records from the Trump transition that were transferred to its facilities after Trump's inauguration.

In another development, members of Trump's transition team that served him after he was elected in November until he took office in January have been ordered to preserve materials related to the Russian Federation matter, the New York Times reported.

The order by 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.

Trump tweeted out Friday morning that his alleged campaign collusion with Russian Federation had been investigated for months but no conclusive evidence had been found.

The Twitter attacks came as vice president Mike Pence hired a personal lawyer to represent him in the intensifying investigation.

Trump's Twitter attack comes a day after Rosenstein issued a cryptic statement, urging Americans not to believe news stories based on anonymous sources.