"Public trust is on the line and the country deserves answers sooner rather than later", he said, adding that Congress needs "all of the facts surrounding the firing of FBI Director Comey".
Meanwhile, House Democratic leader Nancy Pelosi is urging Democrats to sign a discharge petition to establish an independent commission to investigate Russian interference in the presidential election and whether associates of the president co-operated with that effort.
"My decision [to appoint a special counsel] is not the finding that crimes have been committed or that any prosecution is warranted".
Deputy Attorney General Rod Rosenstein said he made this decision out of the enormous "public interest" in this investigation.
Special counsels are appointed to avoid potential conflicts of interest in Justice Department investigations.
The White House maintains that the memo is not an accurate portrayal of the conversation between Mr Trump and Mr Comey. Those calls grew louder as a number of bombshell news reports emerged raising questions about Trump's dealings with Russian Federation and Comey.
Marcus: Why didn't Trump want anyone to hear his conversation with Comey?
Striking a defiant stance, he added: "You can't let the critics and the naysayers get in the way of your dreams". He said he would give the Federal Bureau of Investigation a week and then "if we need a subpoena, we'll do it".
And then there was the story that he had tried to stop the FBI investigation into his former national security adviser.
Mueller was Comey's immediate predecessor at the Federal Bureau of Investigation, which he led from 2001 to 2013 under both George W. Bush and Barack Obama.
Multiple government bodies have been conducting inquiries into Russia's meddling in the 2016 U.S. presidential election and potential instances of collusion between the Kremlin and Trump associates.
Comey was abruptly fired May 9 and learned of the dismissal as he was giving a talk in Los Angeles.
Spicer was also asked about those saying Trump current scandal is of Watergate proportions, and also about Vladimir Putin's offer to release a transcript of what Trump told Foreign Minister Sergey Lavrov at the White House last week.
"A criminal case would require proving that Trump acted corruptly with the specific intent of interfering with the investigation" the attorneys write on Lawfare Blog.
EEUU congela bienes de 8 jueces del TSJ venozalono
La crisis en Venezuela parece estar cambiando posturas. "Lo que está sucediendo en Venezuela es muy, muy triste", dijo Trump.
Allegations that Trump interfered with an FBI investigation stemmed from a New York Times report Tuesday that Comey had documentation of a February conversation between him and Trump in which the president urged Comey to discontinue the investigation of Flynn.
"Israel has full confidence in our intelligence-sharing relationship with the United States and looks forward to deepening that relationship in the years ahead under President Trump", said Israel's ambassador to the US, Ron Dermer.
Two Democrats in the House of Representatives, Maxine Waters and Al Green, have called for articles of impeachment to be drawn up, but most lawmakers are holding fire, fearing it could transform the process into a purely partisan exercise.
Lawmakers on Capitol Hill generally welcomed the Justice Department action and praised Mr. Mueller for his integrity, but House and Senate Republican leaders said they would go on with their own investigations of the Russian Federation matter.
"We must continue to follow the facts wherever they lead so that we can get to the bottom of this", he said.
The Department of Justice working under Trump is unlikely to prosecute the president, but Congress can classify an obstruction of justice as a high crime, or impeachable offense.
Bitdefender blocks WannaCry ransomware attack
This isn't the first time United States spy agencies have been accused of knowing about vulnerabilities and keeping them secret. The malware behind WannaCry (also called Wana Decryptor or WCry) was reported to have been stolen from the NSA back in April.