The novel coronavirus disease likely developed naturally — jumping from an animal to humans, while the possibility of a leak is still not out of doubt, the director of the US National Institutes of Health has said.

It is still unknown if the virus leaked out of a Wuhan lab, Dr. Francis Collins was quoted as saying in an interview on CNBC’s “Squawk Box” on Monday.

Collins said the possibility that scientists at the Wuhan Institute of Virology in China were secretly studying the virus and that it leaked out from there can be not ruled out.

“The vast evidence from other perspectives says no, this was a naturally occurring virus,” he said. “Not to say that it could not have been under study secretly at the Wuhan Institute of Virology and got out of there, we don’t know about that. But the virus itself does not have the earmarks of having been created intentionally by human work.”

The World Health Organisation (WHO) in March concluded that a laboratory leak was “extremely unlikely”. Undermining its own report, Director-General Tedros Adhanom Ghebreyesus last month, proposed a second phase of studies in Wuhan. However, China rejected the probe, accusing the WHO of “arrogance” and a “disrespect for common sense”.

China’s refusal to participate has made the WHO investigation harder, Collins said. “I think China basically refused to consider another WHO investigation and just said ‘nope, not interested,” he said.

“Wouldn’t it be good if they’d actually open up their lab books and let us know what they were actually doing there and find out more about those cases of people who got sick in November of 2019 about which we really don’t know enough,” Collins added.

US intelligence reports first reported by the Wall Street Journal indicated that in November 2019, three workers at the Wuhan Institute of Virology fell ill with symptoms similar to those seen in Covid-19 infections, a report that China said was “completely untrue.”

Collins also pointed to an impending report by US President Joe Biden’s intelligence community, who were given 90 days to further investigate the virus’ origins and report the findings.

While most of the information gathered will likely remain classified, some will be released, according to Collins. “We don’t know what they’re going to come up with either, but we’re intensely interested,” Collins said.