Dr. Anthony Fauci, the nation’s top infectious disease expert, angrily confronted Kentucky GOP Sen. Rand Paul on Tuesday in testimony on Capitol Hill, rejecting Paul’s insinuation that the U.S. helped fund research at a Chinese lab that could have sparked the COVID-19 outbreak.

Paul suggested that Fauci had lied before Congress when in May he denied that the National Institutes of Health funded so-called “gain of function” research — the practice of enhancing a virus in a lab to study its potential impact in the real world — at a Wuhan virology lab. U.S. intelligence agencies are currently exploring theories that an accidental leak from that lab could have led to the global pandemic.

“I have not lied before Congress. I have never lied. Certainly not before Congress. Case closed,” Fauci told Paul before the Senate Health, Education, Labor, and Pensions Committee, saying a study the senator mentioned referenced a different sort of virus entirely from the one responsible for the coronavirus outbreak.

“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.”

He added, “If anybody is lying here, senator, it is you.”

It was the latest in a series of clashes between Paul and Fauci over the origins of the virus that caused the global pandemic.