Dr. Anthony Fauci and Sen. Rand Paul got into heated exchange over the origins of coronavirus.

The tense back-and-forth took place Tuesday during a hearing on Capitol Hill, where the Kentucky GOP senator accused Fauci of lying to Congress during his testimony back in May. Paul pointed out that Fauci previously denied the claims that the National Institutes of Health helped fund controversial research at the Wuhan Institute of Virology, the facility at the center of unfounded lab leak claims. The Republican lawmaker specifically pointed to a NIH-funded study in which Wuhan lab staffers researched coronaviruses in bats using “gain-of-function” methods, which aim to increase the transmissibility of potential pandemic pathogens.

On May 19, the NIH released a statement confirming it had supported grants that went toward disease and virus research in bats and other animals; however, the institute claimed it “never approved any grant that would have supported ‘gain-of-function’ research on coronaviruses that would have increased their transmissibility or lethality for humans.”

Paul wasn’t convinced by the NIH’s denial, and has consistently suggested that the U.S. and the National Institute of Allergy and Infectious Diseases played a significant role in the deadly outbreak.

“Dr. Fauci, as you’re aware, it is a crime to lie to Congress,” Paul said. “On May 11, you stated that the NIH has not ever, and does not now fund gain of function research and the Wuhan Institute of Virology. And yet, gain of function research was done entirely in the Wuhan Institute … and was funded by NIH.”

Paul then asked Fauci if he wanted to retract his initial denial, prompting Fauci to fire back with his own allegations.

“Senator Paul, you do not know what you’re talking about, quite frankly,” Fauci said. “And I want to say that officially. You do not know what you’re talking about … If anybody is lying here, senator, it is you.”

Paul accused Fauci of “trying to obscure responsibility for 4 million people dying around the world from a pandemic.” Fauci fired back, saying he resented “the lie” Paul was propagating.

“If you look at the viruses that were used in the experiments, that were given in the annual reports, that were published in the literature, it is molecularly impossible” for them to caused the pandemic.

“You are obviously obfuscating the truth,” Paul interjected.

“I’m not obfuscating the truth. You are ...” Fauci responded. “I have not lied before Congress. I have never lied, certainly not before Congress. Case closed.”