When the CNN/Des Moines Register debate concluded, the studio audience and viewers saw Warren walk over to Sanders and not shake his outstretched hand.
But sound of the moment was caught by CNN's microphones and found Wednesday.
Sanders disputed that claim before and during the debate but Warren insists it's true. "And I don't want to waste a whole lot of time on this, because this is what Donald Trump and maybe some of the media want".
That denial apparently fell flat as Phillip turned to Warren to ask: "Senator Warren, what did you think when Senator Sanders told you a woman could not win the election?".
Sanders: "You know, let's not do it right now. If you want to have that discussion, we'll have that discussion".
Warren and Sanders, both progressives, had been abiding by an informal non-aggression pact for the entire campaign, but with less than three weeks until the first nomination contest in Iowa, and locked in a tight race, that pledge to go easy on each other now appears over.
Sanders said: "You called me a liar. You told me - all right, let's not do it now". "I just want to say "hi" to Bernie".
The two candidates have largely been seen as allies in the U.S. Senate during their tenures. Warren even declared at a past debate "I'm with Bernie" in a discussion about healthcare.
"Elizabeth Warren is a very good friend of mine", Sanders told the New York Times.
"The only people on this stage who have won every single election that they've been in are the women", Warren responded, her comments drawing thunderous applause. The fact that CNN broke the initial story about the alleged conversation between the candidates, first based on anonymous sourcing but then confirmed by Warren, added another layer of intrigue. Warren said during Tuesday's debate she "disagreed" when Sanders allegedly said this, although she wasn't directly asked whether he did.