Citing a source close to the campaign, NBC News said the campaign turned over information to agents looking into the publication of an audio recording of McConnell and campaign officials discussing opposition research and potential political attacks on actress Ashley Judd, who -- until last month -- had been considered likely to seek the Democratic nomination in Kentucky's 2014 Senate election.
McConnell campaign manager Jesse Benton told former Arkansas Gov. Mike Huckabee in a radio interview Wednesday the FBI is conducting an "an ongoing criminal investigation" and has told the campaign "they're running down some leads," NBC reported.
"We can confirm that Senator McConnell's office reported this matter to us and we are looking into it," FBI spokesman Paul Bresson was quoted by USA Today as saying. "We are reviewing the tape to determine if any federal laws were violated."
The 12-minute audiotape, posted Tuesday on the website of liberal magazine Mother Jones, depicts McConnell aides discussing Judd and her struggles with depression and views on several topics, including religion.
The actress-activist daughter of country music singer Naomi Judd and half-sister of Wynonna Judd flirted with a 2014 Senate bid in Kentucky as a Democrat against McConnell, the Senate minority leader, but said in late March she would not run.
McConnell is heard in the beginning of the recording, in which a wide variety of opposition research on Judd is discussed.
At one point, a McConnell aide is heard saying Judd is "clearly -- this sounds extreme -- but she is emotionally unbalanced. I mean it's been documented. Jesse can go in chapter and verse from her autobiography about, you know, she's suffered some suicidal tendencies. She was hospitalized for 42 days when she had a mental breakdown in the '90s."
An aide also says Judd is critical of "traditional Christianity," the tape indicated.
McConnell Tuesday blamed the "political left" for the anonymous recording.
"Last month, they were attacking my wife's ethnicity. And unbeknownst to me, they were bugging our headquarters -- quite a Nixonian move," McConnell told reporters. "This is what you get from the political left in America these days."
Benton said in a statement, "We've always said the left would stop at nothing to attack Senator McConnell, but Watergate-style tactics to bug campaign headquarters are above and beyond."
Mother Jones -- which created a stir last year when it published a secretly taped video of GOP presidential nominee Mitt Romney's "47 percent" comments -- said it got the recording of the Feb. 2 meeting last week from a source who requested anonymity.
David Corn, who wrote the story, said in a statement the magazine was "not involved in the making of the tape" and rejected the Watergate-style characterization.
Corn, who also reported on Romney's comments, said he and the magazine were still waiting for McConnell to comment on the "substance of the story."
He said he contacted McConnell's Senate and campaign offices and received no response.
Judd spokeswoman Cara Tripicchio said the recording was "another example of the politics of personal destruction that embody Mitch McConnell and are pervasive in Washington."