Dr. Anthony Fauci, the nation's top infectious disease official, said he and many experts felt it was more likely that COVID-19 was a "natural occurrence" but couldn't know the origin for sure, and called for further investigation.

ANTHONY FAUCI: "Because we don't know 100% what the origin is, it's imperative that we look and we do an investigation."

Top U.S. infectious disease expert Dr. Anthony Fauci called for further investigation into the origins of the coronavirus on Tuesday, after a recent Wall Street Journal report about illnesses at a Chinese virology lab raised new questions about where COVID-19 may have come from, but said he and other experts felt it was more likely naturally occurring.

FAUCI: "Many of us feel that it is more likely that this is a natural occurrence, as has happened with SARS-CoV-1, where it goes from an animal reservoir to a human. But we don't know 100% the answer to that."

At the same White House COVID-19 briefing, senior adviser Andy Slavitt said finding the origin of the virus that has killed a reported three and half million people worldwide was a "critical priority."

SLAVITT: "We need to get to the bottom of this and we need a completely transparent process from China. We need the WHO to assist in that matter. We don't feel like we have that now. We need to get to the bottom of this, whatever the answer may be."

Earlier on Tuesday, U.S. health secretary Xavier Becerra called for international experts to be allowed to further evaluate the source of COVID-19 in a second phase investigation during a video address to the World Health Organization, but did not mention China directly.

BECERRA: "Phase 2 of the COVID origins study must be launched with terms of reference that are transparent, science-based, and give international experts the independence to fully assess the source of the virus and the early days of the outbreak."

Government sources have told Reuters that U.S. intelligence agencies are examining reports that three researchers at China's Wuhan Institute of Virology were seriously ill a month before the first cases of COVID-19 were reported.

On Monday, China's foreign ministry said it was "completely untrue" that three staff members at the Wuhan lab had fallen ill.

Meanwhile, two Congressional officials have told Reuters that the Senate and House of Representatives intelligence committees are conducting their own investigations into the origins of COVID-19 AND how the Trump administration responded to the crisis.

Last week, the House committee's Republican minority issued a report focusing particularly on the Wuhan Institute of Virology, asserting that there was "significant circumstantial evidence" that the COVID-19 outbreak may have been the result of a leak from the lab.

RELATEDHow can we stop the next pandemic? Here's what WHO panel recommends