Former White House Press Secretary Ari Fleischer said Thursday he believes it's highly unlikely that President Obama did not know an FBI informant was in the Trump campaign.

"We need to know why did it begin, who authorized it and what role did Barack Obama have. Did he know the FBI had informants there? I'll guarantee you the answer is yes. No FBI would put informants in another presidential campaign without permission from the White House, including the president," he said on "Outnumbered."

He said the FBI counterintelligence probe "spiraled out of control" after the Obama administration chose not to warn Trump about possible infiltration by Russia.

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President Trump has insisted that the FBI placed a "spy" in his campaign, calling it the "one of the biggest political scandals in U.S. history."

Clapper has now admitted that there was Spying in my campaign. Large dollars were paid to the Spy, far beyond normal. Starting to look like one of the biggest political scandals in U.S. history. SPYGATE - a terrible thing! — Donald J. Trump (@realDonaldTrump) May 24, 2018

Not surprisingly, the GREAT Men & Women of the FBI are starting to speak out against Comey, McCabe and all of the political corruption and poor leadership found within the top ranks of the FBI. Comey was a terrible and corrupt leader who inflicted great pain on the FBI! #SPYGATE — Donald J. Trump (@realDonaldTrump) May 24, 2018

Former FBI Director James Comey and former Director of National Intelligence James Clapper have pushed back on his claims, with Clapper saying that an informant would have been targeting the Russians, not Trump.

Facts matter. The FBI’s use of Confidential Human Sources (the actual term) is tightly regulated and essential to protecting the country. Attacks on the FBI and lying about its work will do lasting damage to our country. How will Republicans explain this to their grandchildren? — James Comey (@Comey) May 23, 2018

FBI and Justice Department officials briefed bipartisan group of lawmakers known as the "Gang of 8" Thursday on classified documents related to the special counsel's Russia investigation, after meeting with two key House Republican lawmakers.

Fleischer emphasized that he favors a full investigation of Russia's meddling in the election, but the public needs answers on the FBI informant's role and other key details.

"It is fraught with risk when you put informants into an opposing political campaign," he explained.

Watch the full discussion above.

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