EXCLUSIVE: Sen. Marsha Blackburn, R-Tenn., has written to President Biden to press him about ongoing concerns with the World Health Organization's investigation into the origins of the COVID-19 outbreak -- amid renewed scrutiny of the Wuhan Institute of Virology, where some believe the pandemic may have originated.

In a letter to Biden, Blackburn highlights "lingering concerns about the validity of the [WHO’s] investigation into the origin of the COVID-19 pandemic."

US LEARNED SEVERAL WUHAN LAB RESEARCHERS SICKENED BEFORE COVID-19 OUTBREAK: FORMER STATE DEPARTMENT OFFICIAL 

Blackburn had written to Biden in March about concerns about the WHO probe, including that certain investigators may have had conflicts of interest and longstanding concerns about Chinese pressure on the investigators.

The initial team of investigators concluded that it was "extremely unlikely" that the virus escaped from the Wuhan Institute of Virology, saying instead that it was more likely that it transmitted from an animal to a human. The Biden administration at the time expressed "deep concerns" about the process for how the team reached the findings, and how they were communicated.

Last month, the WHO released a report from a joint international and Chinese government investigation expressing similar conclusions. However, other sources have emerged that challenge that conclusion -- including a Wall Street Journal report that several researchers from the Wuhan institute became so sick in Nov. 2019 that they sought hospital care.

WHO UNDER FIRE FOR CONCLUDING COVID-19 HIGHLY UNLIKELY TO HAVE COME FROM WUHAN LAB 

China accused the U.S. on Sunday of continuing its effort to "hype the lab leak theory."

Blackburn notes concerns from scientists that only four of the 313 page report addresses the possibility of a lab leak, and goes on to ask a number of questions of Biden, including asking what the WHO gave as reasons for rejecting three scientists recommended by the U.S. for the team that went to Wuhan.

She also notes that the joint investigation says it lacked the expertise to investigate a lab leak theory, and asks what the administration is doing so that "competent, impartial experts are involved in future investigations?"

CLICK HERE TO GET THE FOX NEWS APP

White House Press Secretary Jen Psaki on Monday said the U.S. has repeatedly called for the WHO to support "an expert-driven evaluation of the pandemic’s origins that is free from interference or politicization." 

"During that first phase of the investigation, there was not access to data, there was not information provided," she said. "And now we’re hopeful that WHO can move into a more transparent, independent phase two investigation."