Ok, here are the facts from the link:

1) Peter Daszak, president of EcoHealth Alliance, was involved with viral research at the WIV. He received a grant from the NIH titled "Understanding the Risk of Bat Coronavirus Emergence." The grant was for $3.7 million. $600,000 went to the WIV who was a key collaborator in the work. The grant was controversial and was suspended in July 2020.

2) At the beginning of the pandemic, Daszak organized a letter in the Lancet that sought to present the lab-leak hypothesis as a groundless and destructive conspiracy theory. Daszak was later dismissed from the Lancet's COVID-19 commission for refusing to share progress reports from his research grant.

3) Daszak's grant set off alarm bells in 2016 when he filed a progress report that stated that scientists planned to create an infectious clone of Middle East Respiratory Syndrome (MERS), a novel coronavirus found in dromedaries that had emerged in Saudi Arabia in 2012. The report also made clear that the NIH grant had already been used to construct two chimeric coronaviruses similar to the one that caused Severe Acute Respiratory Syndrome (SARS), which emerged in 2002 and went on to cause at least 774 deaths worldwide.

4) Obama put a moratorium on gain-of-function research, but Daszak continued his work, stating that SARS-like chimeras from the completed experiment were exempt from the moratorium, because the strains used had not previously been known to infect humans. He also pointed to a 2015 research paper in which scientists had infected humanized mice with the same strains, and found that they were less lethal than the original SARS virus. This chimeric work took place at the WIV. Declassified intelligence stated that the Chinese military had also been working with civilian scientists at the WIV since 2017.

5) Needing funding, Daszak and his collaborators (EcoHealth Alliance and scientists at WIV) applied for a DARPA grant in 2017. In the leaked proposal, there was "a plan to examine SARS-like bat coronaviruses for furin cleavage sites and possibly insert new ones that would enable them to infect human cells." This is particularly notable because the COVID-19 virus has a unique furin cleavage site. The grant application proposed to collect bat samples from caves in Yunnan Province, transport them to the Wuhan Institute of Virology, extract and manipulate the viruses they contain, and use them to infect mice with humanized lungs. It would then map high-risk areas for bats harboring dangerous pathogens and treat test caves with substances to reduce the amount of virus they were shedding. The contract was “never awarded because of the horrific lack of common sense.” DARPA viewed the EcoHealth Alliance as a middle-man willing to travel to China, and nothing more.

6) As COVID-19 started spreading in 2019, to Dr. Robert Redfield, the director of the CDC at the time, it seemed not only possible but likely that the virus had originated in a lab. “I personally felt it wasn’t biologically plausible that [SARS CoV-2] went from bats to humans through an [intermediate] animal and became one of the most infectious viruses to humans,” he told Vanity Fair.

7) Early on in the pandemic, Fauci organized a small group of scientists to quell the lab-leak theory despite privately having similar concerns in emails obtained via FOIA requests. Dr. Robert Redford urged investigation of a possible lab leak days before Fauci's meeting occurred. Part of Fauci's group was scientist Kristian Andersen, who would later publish a preprint implicating the wet market as the sole origin of the virus.

8) Later, a scientist Jesse Bloom created a pre-print paper that investigated the disappearance of viral genetic sequences mentioned in early SARS-CoV-2 papers. The NIH had deleted these at the request of the WIV. Bloom invited Fauci and NIH director Francis Collins to discuss the paper. Collins invited outside biologist Kristian Andersen (mentioned above) to the meeting. Andersen was aggressive during the meeting and said it was the right of the WIV to deleted their samples. Andersen had access to the preprint server, and offered to entirely delete the paper or to revise it "in a way that would leave no record that this had been done." Fauci responded to this by saying: "Just for the record, I want to be clear that I never suggested you delete or revise the pre-print."

That's the general gist of the circumstantial evidence. The tl;dr is the U.S. (and likely the Chinese military) was funding gain-of-function research on this exact type of virus at the WIV. Those involved with the project have been the quickest to dismiss the possibility of a lab leak while simultaneously refusing to share their research. There also isn't enough evidence to conclude a natural origin as a similar virus has not been found in the wild. The furin cleavage site, in particular, is unique to COVID-19, and this was the specific target of gain-of-function research at WIV.