Trump's "spy" comments marked his latest attack on the FBI at a time when he feels mounting pressure from the ongoing investigation by Mueller, appointed previous year by the Justice Department, into potential collusion between his 2016 campaign and Russian Federation and whether he has committed obstruction of justice.
On whether there was an "informant" in the 2016 presidential campaign, Giuliani told CNN: "I don't know for sure, nor does the president, if there really was one", though he said they have always been told there was "some kind of infiltration".
"Nothing we heard today has changed our view that there is no evidence to support any allegation that the Federal Bureau of Investigation or any intelligence agency placed a "spy" in the Trump campaign, or otherwise failed to follow appropriate procedures and protocols", the Democrats said in a joint statement.
Giuliani adds the White House may urge the Justice Department "to re-evaluate" the investigation. Ryan will not attend, due to a "longstanding schedule commitment", according to spokeswoman AshLee Strong, and three Senate Judiciary Committee Republicans - Chuck Grassley (Iowa), John Cornyn (Texas), and Lindsay Graham (S.C.) - have also asked to attend.
As cynical as they might sound about Washington's capacity for reform - even in the likes of Mueller, a staunch Republican with a stellar public record - Trump supporters do care about accountability.
Mr. Ryan, who has been criticized for not reining in House Republican attacks on the Russian Federation inquiry and federal law enforcement, defended the unusual meetings. Negotiations over the information stalled, but restarted when Trump demanded in a tweet Sunday that the Justice Department investigate "whether or not the FBI/DOJ infiltrated or surveilled the Trump Campaign for Political Purposes".
While Yates would not comment on the allegations of spying, deferring comment on the investigation to the Justice Department and FBI, she said she was confident the investigation would bear out the truth.
The White House has yet to provide any evidence to support Trump's claim that President Barack Obama's administration was trying to spy on his 2016 campaign for political reasons - and it remained unclear whether any spying took place.
Former FBI Director James Comey, who was sacked by Trump past year, tweeted Wednesday morning that the agency's use of secret informants was "tightly regulated and essential to protecting the country".
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The Justice Department had rejected Nunes' original request, writing in a letter in April that his request for information could put lives in danger.
So far, 19 people, including Trump's former campaign chairman and former national security adviser, have been charged in Mueller's investigation.
"If they insist upon carrying out this farce, the White House and its Republican allies in the House will do permanent, longstanding damage to the practice of bipartisan congressional oversight of intelligence", Warner said.
This is not the first time Trump has made broad claims about the investigation.
Trump on Friday issued a string of accusatory Twitter messages asserting that the use of the informant was part of a political plot against him.
Explaining a new policy aimed at stopping "piling on"-situations where companies are punished a couple of times for the same crime by different government agencies-he said: "The dictionary defines "piling on" as joining other people in criticizing someone, usually in an unfair manner".
Trump has long argued that both the FBI and Mueller's investigations into Russian meddling and possible collusion with his campaign in the 2016 election are "phony", a "witch hunt" "and a "hoax".
"There's never been a Gang of Eight meeting with that kind of White House presence", he said.
But Trump lawyer Rudy Giuliani cast doubt on that.