Former FBI Director James Comey must testify before the Senate Judiciary Committee as soon as possible, US Senator Lindsey Graham said in a statement on Wednesday.
The White House says Comey's contemporaneous recollection of his discussions with Trump are not accurate, while some legal experts say that if true, Trump's efforts to end the Flynn probe could be construed as obstruction of justice, an impeachable offense.
Given that Comey reportedly made a habit of taking notes, the committee also asked the FBI for any notes Comey took from interactions with former President Barack Obama, Attorney General Jeff Sessions, former Attorney General Loretta Lynch, Deputy Attorney General Rod Rosenstein and former Deputy Attorney General Sally Yates, regarding the Clinton email investigation or Trump associates' alleged ties to Russian Federation.
Part of Mueller's charge is to determine whether any of Trump's campaign aides actively cooperated with Russian intelligence efforts to influence the USA election a year ago.
Meanwhile, Reuters reported that Flynn and other Trump aides were in contact with Russian officials and others with Kremlin ties in at least 18 calls and emails in the last seven months of the presidential campaign ahead of the November 8 vote.
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Rep. Jason Chaffetz, Republican chairman of the House Oversight Committee, sent a letter to the FBI Tuesday requesting that it turn over all documents and recordings that detail communications between Comey and Trump.
Trump himself hasn't directly addressed the latest allegations that he pressured Comey to drop the Flynn investigation.
Trump later acknowledged that he had chose to fire Comey regardless of their recommendation because of what he called "the Russian Federation thing".
Under the law, Mueller will have authority to choose his own staff and request his own budget from the Justice Department.
Later, Flynn emerged as a sharp critic of Obama's handling of national security matters, while supporting Trump's long-shot run for the presidency. It described the president's request the Comey end an investigation into former national security director Michael Flynn's ties to Russian Federation and Turkey.
"If you ask me", he said, "one of the main problems in Washington, D.C., is everybody is so busy running around trying to protect their reputation instead of protecting the republic, which is what they're supposed to be doing".