He tweeted: "Wow, word seems to be coming out that the Obama FBI SPIED ON THE TRUMP CAMPAIGN WITH AN EMBEDDED INFORMANT".
But claims by Trump allies there was an informant embedded inside the campaign have not been substantiated.
The F.B.I. obtained phone records and other documents using national security letters - a secret type of subpoena - officials said.
McCarthy said "there is probably no doubt" that there was at least one confidential informant in the Trump campaign.
Ultimately, however, there appears to be "virtually no evidence" tying the Trump campaign "to any cyberespionage by Russia", McCarthy said. "If there's a spy, they got nothing from it".More news: Microsoft to Launch Cheaper Tablet Line to Rival Apple's iPad
In addition to all of the accusations of espionage, Trump also called the investigation into the possible collusion between his 2016 Presidential election campaign and the Russian government (and the potential shady ties therein) by the FBI's Special Counsel Robert Mueller "the greatest witch hunt in American history".
"I've written a couple of columns in the last week or so pointing out that there is probably no doubt that [the FBI] had at least one confidential informant in the campaign", said McCarthy (pictured above right), a National Review contributor and former assistant US attorney for the Southern District of NY.
Speaking Thursday on Fox and Friends, the former NY mayor said Mr Trump would only sit down with Mr Mueller if "we feel there's a way to shorten this thing".
On Wednesday night, Trump's newest lawyer Rudy Giuliani was talking about it on Fox News, telling Laura Ingraham that the Federal Bureau of Investigation "possibly plac [ed] a spy in the Trump campaign".
If Comey was involved, said Giuliani, "he should be prosecuted... We've tortured this president enough", he added, describing the investigation as being "like a big weight" on the president's back. Trump's former national security adviser, Michael Flynn, and his deputy campaign chairman, Rick Gates, have pleaded guilty and are now cooperating with the probe.