Sen. Tom Cotton said Dr. Anthony Fauci and the National Institutes of Health (NIH) need to "come clean" about providing funding for a Wuhan lab in China, as the GOP senator demanded a "credible" investigation into the origins of the coronavirus pandemic. 

Cotton, R-Ark., expressed doubt that the taxpayer money that went to the Wuhan Institute of Virology to study coronaviruses did not support "gain of function" research, which involves modifying a virus to make it more infectious among humans. Fauci, the head of the National Institute of Allergy and Infectious Diseases (NIAID), has said the money was not for gain of function research, but rather studying bats. 

"I hope Tony Fauci gives some straight answers today for once," Cotton told "America's Newsroom" on Wednesday in advance of the doctor's testimony before a Senate committee. 

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"They need to come clean with the American people, they need to stop playing word games," Cotton continued. "They need to explain why they funded that research apparently in violation of at least the spirit, if not the letter, of an Obama White House directive."

Cotton, Sen. Rand Paul, R-Ky., and other Republicans are concerned about a $600,000 grant from NIAID that went to a group called EcoHealth Alliance, which then paid the Wuhan Institute of Virology to study the risk that bat coronaviruses could infect humans. 

Fauci on Tuesday defended the "modest" collaboration with scientists in Wuhan, saying it would have been "almost a dereliction of our duty if we didn't study this, and the only way you can study these things is you've got to go where the action is," referencing the early-2000s SARS outbreak, which is presumed to have come from bats. 

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"You don't want to study bats in Fairfax County, Virginia, to find out what the animal-human interface is that might lead to a jumping of species," Fauci continued. "So we had a modest collaboration with very respectable Chinese scientists who were world experts on coronavirus, and we did that through a sub-grant from a larger grant to EcoHealth."

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But under questioning by Sen. John Kennedy, R-La., Wednesday during a Senate Appropriations subcommittee hearing, Fauci acknowledged that he's relying on the statements and research from the "competent trustworthy scientists" in China who said they did not fund gain of function research with the U.S. grant money. 

"Have you ever had a grantee lie to you?" Kennedy asked. 

"I cannot guarantee that a grantee has not lied to us, because you never know," Fauci responded. 

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Cotton and other Republicans want an independent investigation into whether COVID-19 was man-made and escaped the Wuhan laboratory or whether the virus occurred naturally and jumped from animals to humans. They've panned the World Health Organization report with Chinese scientists in March that found the virus most likely originated from bats and downplayed the lab leak theory as "extremely unlikely."

"What matters most to me is that the American people get answers," Cotton said. "Was the Chinese government responsible for unleashing the worst pandemic in a century on the world through a combination of its negligence in these labs and then its deceitfulness in covering it all up?

"That's why we need a credible investigation, not another whitewash by the World Health Organization," Cotton added.

Fox News' Tyler Olson contributed to this report.