Progress Kentucky Executive Director Shawn Reilly and Curtis Morrison, a former spokesman for the local liberal political action committee, recorded the conversation while in a hallway of McConnell's re-election headquarters in Louisville Feb. 2 after an open house inaugurating the headquarters, said Jacob Conway, a Democrat active in Louisville politics.
"Apparently the gentlemen overheard the conversation and decided to record it with a phone or recording device they had in their pocket," Conway told National Public Radio affiliate WFPL, Louisville.
"They were in the hallway after the -- I guess after the celebration and hoopla ended. Apparently these people broke for lunch and had a strategy meeting," Conway said. "One of them held the elevator, the other one did the recording, and they left."
A lawyer for Reilly told The Washington Post his client was at the headquarters at the time but did not commit any crime.
"Our client is at most a witness to potential criminal activity," Ted Shouse told the newspaper.
Morrison, reached through Facebook by The (Louisville) Courier-Journal, said he was not doing interviews and declined to say if he had hired a lawyer, if he had been interviewed by the FBI or if he had recorded the McConnell meeting.
In the recording, first posted online by Mother Jones magazine Tuesday, McConnell, who is up for re-election in 2014, tells aides "this is the Whac-a-Mole period of the campaign," adding, "when anybody sticks their head up, do them out."
Whac-a-Mole is an arcade game.
Most of the recording depicts staff members discussing negative research about actress-activist Ashley Judd, who flirted with the idea of running as a Democrat against McConnell next year but said in late March she would not run.
Progress Kentucky says on its website its mission is to elect a new U.S. senator to represent Kentucky.
"It's time to level the playing field. It's time to make things right. It's time to hold Mitch McConnell accountable," says the site, reviewed by United Press International.
Conway told Fox News Channel he came forward because he wanted to protect the local and state Democratic Party from being blamed for the recording after the FBI began looking into it. Progress Kentucky is unaffiliated with Kentucky's Democratic Party.
Progress Kentucky's treasurer resigned after the audio was published, NBC News reported late Thursday.
"At this time, based on advice of both friends and counsel, I will be not be making a public statement available until everything has been reviewed by an attorney at this time," Douglas L. Davis told the network.
"I have resigned my position as treasurer and did not and do not condone any allegations of illegal activity that might have taken place," he said.
Davis is the second official to quit in two months.
Morrison quit as the group's spokesman in February after the super PAC's Twitter account was linked to racially charged comments about McConnell's wife, Elaine Chao, who was the U.S. labor secretary in the George W. Bush administration. She was born in Taipei, Taiwan.