Sanders responded that it was Warren who called him a liar and said they should not talk about it right then.
But sound of the moment was caught by CNN’s microphones and found Wednesday.
“I think you called me a liar on national TV,” Warren can be heard saying.
“What?” Sanders responded.
“I think you called me a liar on national TV,” she repeated.
“You know, let’s not do it right now. If you want to have that discussion, we’ll have that discussion,” Sanders said, to which Warren replied, “Anytime.”
“You called me a liar,” Sanders continued. “You told me — all right, let’s not do it now.”
A spokesperson for the Sanders campaign declined to comment on the audio. A spokesperson for Warren’s campaign did not respond to a request for comment.
The tense interaction between Sanders and Warren capped a 48-hour period in which the two progressive presidential front-runners, who have mostly gone through a year-long campaign without any public signs of discord, were suddenly put at odds.
Sanders has repeatedly denied making the comment while Warren has held firm in saying that he did — and that she disagreed with his assessment.
The terse post-debate exchange between Warren and Sanders was all the more striking because it came during a routine round of pleasantries among the rivals, as the crowd’s applause still echoed inside the Sheslow Auditorium at Drake University in Des Moines.
“Joe, good job,” Warren said, extending her hand to former Vice President Joe Biden. “Good to see you.”
“Good job, Pete (Buttigieg),” Sanders said. “Good job, Amy (Klobuchar).”
Then, Warren came face-to-face with Sanders and confronted him, puncturing the non-aggression pact that has held for the past year.
It was notable that before the debate, the two senators shook hands when they arrived on stage. When they left, they did not.
The conversation was not captured on the primary audio feed from the candidates’ podiums. After the debate, CNN did an inventory of the audio equipment that was used and found two backup recordings from the microphones Sanders and Warren were wearing.
CNN then synchronized the audio recordings with the footage that was broadcast live on Tuesday night.
The conversation played out on the debate stage, in public view, and occurred before Sanders and Warren’s clip-on microphones were removed.
During Tuesday’s debate, Sanders and Warren both doubled down on their differing accounts of the 2018 meeting.
Pressed by the moderators, Warren called Sanders her “friend” and insisted she wasn’t there “to try to fight with Bernie,” before pivoting to the broader issue of women running for president.
Sanders, who had said he didn’t “want to waste a whole lot of time on this because (a fight between them) is what Donald Trump and maybe some of the media want,” vehemently denied questioning whether a woman could win the presidency.
“Anybody who knows me, knows that it’s incomprehensible that I would think that a woman could not be president to the United States,” he said. “Go to YouTube today. They have some video of me 30 years ago talking about how a woman could become president of the United States.”
CNN’s Gregory Krieg contributed to this report.