Newly released Slack messages cited by Sen. Rand Paul's investigation are fueling fresh scrutiny of the influential scientific paper that dismissed the laboratory-leak theory for the origin of COVID-19.

The messages, highlighted in a report by The Federalist, show several authors of the 2020 "Proximal Origin" paper privately discussing evidence they believed supported a lab-leak scenario, even after publishing the paper, which concluded such an origin was not plausible.

The disclosures expand on earlier messages released by the House Select Subcommittee on the Coronavirus Pandemic in 2023, which showed lead author Kristian Andersen privately describing a lab origin as "friggin' likely."

In another exchange, Andersen wrote, "The main issue is that accidental escape is in fact highly likely — it's not some fringe theory," while another author acknowledged that COVID "easily could have" leaked from a laboratory.

Even so, the paper published in Nature Medicine on March 17, 2020, concluded a laboratory origin was not plausible and quickly became one of the most influential studies cited during the pandemic.

When those earlier messages surfaced, Andersen said they reflected the normal scientific process and that his concerns had been resolved before publication.

According to The Federalist, the latest Slack messages cast doubt on that explanation.

Months after the paper was published, Andersen wrote in a June 2020 message: "Gotta say — some smoking(ish) guns starting to appear that I'm not comfortable with."

Rather than publicly revising the paper's conclusions, The Federalist reported the researchers privately discussed defending the natural-origin theory despite lingering concerns.

The report also revisits the creation of "Proximal Origin," saying it followed a Feb. 1, 2020, conference call organized by then-Director of the National Institute of Allergy and Infectious Diseases Anthony Fauci after questions emerged about whether the virus originated in a laboratory.

According to The Federalist, Fauci's role in helping launch the effort was not disclosed when the paper was published. The report also notes Fauci later pointed to the study after President Donald Trump publicly suggested COVID-19 may have escaped from a lab.

The newly released messages indicate Fauci remained in contact with Andersen after publication.

In a May 23, 2021, Slack message cited by The Federalist, Andersen wrote: "Just had a meeting with Fauci and his team this morning. They get it and deeply appreciate what we're doing, so we'll keep dancing …"

The messages also show the authors revisiting some of the paper's scientific arguments.

In one exchange, Andersen acknowledged: "In our paper we argue against (a) manipulation and (b) tissue culture. Our main arguments were that manipulation is hard. But the German's [sic] came out and created a reverse genetics system of SARS2 in less than two weeks, so clearly it's not that hard."

In another discussion, Andersen reportedly conceded the paper overstated how common certain coronavirus research was, writing: "We describe this in the paper and say that this work is performed 'all of the world' [sic], but that isn't true — it has exclusively been going on at WIV [Wuhan Institute of Virology]."

The new disclosures are expected to add fuel to the long-running debate over COVID-19's origins, as congressional investigators continue examining the pandemic, federal health officials' response, and the scientific research that shaped public policy.