DES MOINES, Iowa (Reuters) - The mystery of what presidential rivals Elizabeth Warren and Bernie Sanders said to each other in a heated exchange after Tuesday night’s Democratic debate has been solved, with debate host CNN revealing that Warren accused Sanders of calling her a liar on national television.

Democratic 2020 U.S. presidential candidates (L-R) Senator Elizabeth Warren (D-MA) speaks with Senator Bernie Sanders (I-VT) as billionaire activist Tom Steyer listens after the seventh Democratic 2020 presidential debate at Drake University in Des Moines, Iowa, U.S., January 14, 2020. REUTERS/Shannon Stapleton

In an exchange caught on camera after the debate but unable to be heard by the television audience, Sanders responded to Warren that it was she who had called him a liar. Moments earlier Warren had refused to shake Sanders’ hand.

The two U.S. senators and liberal standard bearers in the Democrats’ nominating contest to pick a candidate to take on Republican President Donald Trump in November had been locked in a dispute before and during the debate over an allegation by Warren that Sanders had told her in a private 2018 meeting that a woman could not be elected president.

Sanders disputed that claim before and during the debate but Warren insists it’s true.

CNN said on Wednesday that its microphones had caught the post-debate exchange and released its contents.

After failing to shake Sanders’ hands, Warren said: “I think you called me a liar on national TV.”

“What?” Sanders replied.

“I think you called me a liar on national TV,” Warren repeated.

Sanders replied: “You know, let’s not do it right now. If you want to have that discussion, we’ll have that discussion.” Warren said: “Anytime.”

Sanders said: “You called me a liar. You told me – all right, let’s not do it now.”

Another Democratic candidate, billionaire Tom Steyer, who was standing behind the two, said: “I don’t want to get in the middle. I just want to say ‘hi’ to Bernie.”

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.