Here's more from CBS News:
Jesse Benton, a McConnell campaign adviser, blasted "the left" for what he called "Nixonian tactics" in acquiring the recording.
"We've always said the left would stop at nothing to attack Sen. McConnell but Nixonian tactics to bug campaign headquarters is above and beyond," he told CBSNews.com in an email. "This is Watergate tactics."
Benton says the campaign is "working with the FBI" to discern the source of the recording, and he suggested there might be a subsequent criminal investigation.
"Senator McConnell's campaign is working with the FBI and has notified the local U.S. Attorney in Louisville, per FBI request, about these recordings," Benton told CBSNews.com." Obviously a recording device of some kind was placed in Senator McConnell's campaign office without consent. By whom and how that was accomplished will presumably be the subject of a criminal investigation."
CBS also published details of the recorded conversations from the Mother Jones article.
Wouldn't surprise me if it was one of the aides.