The U.S. intelligence community is nearly halfway through a 90-day review of the origins of COVID-19. Investigators are looking at whether the virus emerged naturally from human contact with an infected animal or if it accidentally escaped from a lab in Wuhan, China.

Former State Department officials tell CBS News significant information about the Wuhan lab's activities in 2019 was buried in government databases and didn't reach their desks for nearly a year.

In Southwest China, the nearest known relatives of the COVID-19 virus were discovered in caves nearly a decade ago, more than 900 miles from Wuhan, where the coronavirus pandemic began.

"It's impossible to tell whether that last bit of evolution took place in a laboratory or whether it took place in nature," Stanford Professor and Microbiologist David Relman told CBS News' Catherine Herridge.

Relman is no stranger to complex cases. When Anthrax was sent through the mail in 2001, Relman reviewed the science underpinning the FBI probe. Now, he's advising the government on the science behind the pandemic's origins.

He told CBS News that the lab-leak hypotheses are "absolutely legitimate" and are "plausible." Relman said the Wuhan lab housed a vast library of bat coronaviruses, including specimens from the caves.

"They weren't just studying these viruses. They were actually collecting samples from nature in the largest number with the greatest diversity of almost any place on the planet," he said.

Seventeen months ago, as COVID-19 went global and even less was known about the virus, a group of scientists labeled the lab-leak theory a "conspiracy."

"The letters that made very defiant or firm statements about the strong likelihood of a natural origin. I think it did some real harm. They forced a lot of people into one way of thinking," Relman said.

Dr. Anthony Fauci, who supports the infected animal to human pathway, recently told CBS News an accidental lab leak is worth investigating.

"We always said keep an open mind and continue to look. So I think it is a bit of a distortion to say we deliberately suppressed that," Fauci said. The lab-leak theory gained traction in January after the State Department released a fact sheet accusing the Chinese communist party of "deceit and disinformation." David Feith, a former deputy assistant secretary of state, helped draft the fact sheet. Among the findings, researchers at the Wuhan Institute of Virology became sick with COVID-like symptoms in the fall of 2019 before the Chinese communist party went public with the virus. Feith said the information was solid and credible. "The U.S. government's information on this, on this fact, on the lab workers being ill was completely high confidence," Feith said. State Department investigators also found the Wuhan lab had military ties and did high-risk research on coronaviruses. Feith added that the lab is "hiding lab records, they are hiding medical records." CBS News has also learned that one of the U.S. government's top labs, Lawrence Livermore, issued a classified report last year that found the lab-leak theory was plausible. "We read it. And we were instantly both alarmed and impressed by its quality, but alarmed by its conclusions," said David Asher, who recently headed a coronavirus task force at the State Department. "What I can say is that it was important and persuasive. We needed to do deeper diligence including the potential it came out of a lab," Asher said. When a group of scientists gathered by the World Health Organization went to the Wuhan lab earlier this year, their access was limited. Feith said the scientist's findings are not enough.

"Those findings were completely inadequate. They were not scientific at all. They were not forensic," Feith said.

This lack of transparency rings true to Condoleezza Rice, who was the White House national security adviser at the outset of the SARS outbreak in 2003.

"We couldn't get answers from the Chinese. And so if we're not going to keep repeating this problem, we have to be a little bit more aggressive with the Chinese about the need to cooperate," Rice said.

With nearly four million dead, including over 600,000 Americans, Professor Relman said solving the puzzle of COVID-19 will combat future pandemics.

"The better we understand the origins of this one, the better we can anticipate the origins of the next one," Relman said.

In response to CBS News questions, China's foreign ministry said "the so-called 'China obstructing investigation' by the U.S. State Department is a 'conspiracy theory' concocted by the United States to shirk responsibility in its failed fight against the epidemic. It has no base in reality and it is purely political manipulation."