Security personnel stand guard outside the Wuhan Institute of Virology in Wuhan as members of the World Health Organization (WHO) team investigating the origins of the Covid-19 coronavirus make a visit to the institute in Wuhan in China's central Hubei province on February 3, 2021
Washington CNN  — 

A group of bipartisan lawmakers is urging President Joe Biden to continue investigating the origins of the coronavirus pandemic until the intelligence community has a high degree of confidence in its conclusion about where and how it started, even if that effort exceeds the ongoing 90-day review set to end later this summer, according to a new letter obtained by CNN.

The July 27 letter is signed by bipartisan leaders of the Senate Intelligence Committee, Mark Warner and Marco Rubio, as well as the top lawmakers on Senate Foreign Relations Committee, Sens. Jim Risch and Bob Menendez.

“Identifying where the virus originated and how it first spread will be critical to preventing future pandemics. If the 90-day review you have announced does not yield conclusions in which the United States has a high degree of confidence, we urge you to direct the intelligence community to continue prioritizing this inquiry until such conclusions are possible,” the letter states.

“A full and impartial investigation that carefully considers all credible theories, backed by all available evidence, is critical,” it adds, noting this includes theories of accidental release from a lab and zoonotic spillover, both of which the intelligence community and scientists have said remain viable.

The senators say that the intelligence community should continue to evaluate evidence regarding researchers at the Wuhan Institute of Virology who fell in in the fall of 2019. It should identify other researchers who were working on coronavirus projects and attempts by the Chinese government to silence or disappear them, the letter says.

The review should also consider information about any WIV gain-of-function research specific to coronavirus or other potential human pathogens, laboratory safety standards and details or any research in connected to military work funded by the Chinese government, it adds.

Additionally, the probe should examine any evidence pointing to possible transmission of the SARS-CoV-2 from animals to humans, including zoonotic transmission chains and the most probable timing of a possible spillover event, it states.

Those priorities, including an investigation into efforts by the Chinese government to obscure the origins of the virus, align closely with intelligence collection priorities set forth as part of Biden’s ongoing 90-day review.

The Office of the Director of National Intelligence declined to comment on the letter. CNN has reached out to the White House.

A memo distributed to the intelligence community on June 11 by ODNI, and previously obtained by CNN, tasked officials with obtaining more data on both the animal and lab leak theories, citing “key intelligence needs” that could help determine the origins of the virus.

The memo called for more examination of several topics including any “detections of the virus in wild or domestic animals,” the “breadth of high-risk Chinese and regional wildlife activities,” any viruses that “could serve as the backbone for genetic engineering purposes,” and the “breadth of Chinese biological research and related activities.”

The memo also called on the intelligence community to study the “breadth of Chinese actions that may have inhibited World Health Organization or other origins investigations.”

But while sources have told CNN that investigators currently consider both theories to be credible, some officials have voiced skepticism that the intelligence community will be able to reach a conclusion that yields a high degree of confidence, as requested in Tuesday’s letter from bipartisan lawmakers.

A person familiar with the current probe previously told CNN there will likely not be a definitive answer as to Covid’s origins at the end of the 90-day review – instead, any final assessment will likely lay out both theories and evaluate their pros and cons. The source said the relationship with China is too delicate to make an endorsement of one theory over another without smoking gun evidence, which doesn’t exist right now.

CNN’s Natasha Bertrand and Katie Bo Williams contributed reporting