"This is not considered by the intelligence community...to be a frivolous matter or something that was overblown," Andrew McCabe said.
Mueller will appear before the House Judiciary Committee for three hours and before the House Intelligence Committee for two hours.
"There may be grounds for removal from office or there may be grounds for indictment after he leaves office that the Congress discovers," Schiff said.
"I didn't think this summit went well for the president at all, more importantly for the U.S. national security," Mike Rogers, a former Republican congressman said.
President Donald Trump's former "fixer" Michael Cohen will testify before three congressional committees this week.
Adam Schiff, chairman of the House intelligence committee, said the panel would probe the financial interests of Trump, his family and allies to determine if those interests are driving the president's or his administration's decisions.
The DOJ gave house Republicans thousands of documents.
"We see no reason to dispute the conclusions," Senate Intelligence Committee Chairman Richard Burr stated.
Under pressure, the Justice Department provided access to the original document that launched the investigation into Russian election interference.
Trump welcomed the news.
"Politics beat a desire for the truth," former Attorney General Eric Holder said, slamming House Intelligence Committee Republicans for unilaterally shutting down the Trump-Russia probe.
Hicks resigned from her role on Wednesday.
After a closed-door interview with White House communications director Hope Hicks, Representative Dennis Heck (D-WA) declared: "We got Bannoned!"
Representative Adam Schiff said House Intelligence Committee Chairman Devin Nunes changed a controversial memo after the panel reviewed it.
In a 165-page transcript of testimony, Simpson discusses Trump's shadowy business dealings.
The panel will be meeting with Steve Bannon and FBI and Justice Department employees.
Bannon has agreed to an interview with the committee, which is investigating Russia's election meddling and links to the Trump campaign.