President Joe Biden has ordered intelligence officials to “redouble” efforts to investigate the origins of Covid-19, including the theory that it came from a laboratory in China.

The president said the US intelligence community was split on whether it came from a lab accident or emerged from human contact with an infected animal.

He asked the groups to report back to him within 90 days.

China has rejected the laboratory theory.

“Smear campaigns and blame shifting are making a comeback, and the conspiracy theory of ‘lab leak’ is resurfacing,” its embassy in the US said in a statement on May 27.

Since it was first detected in the Chinese city of Wuhan in late 2019, more than 168 million cases of Covid-19 have been confirmed worldwide and at least 3.5 million deaths reported.

Authorities linked early Covid cases to a seafood market in Wuhan, leading scientists to theorize the virus first passed to humans from animals.

However, recent media reports have suggested growing evidence the virus could instead have emerged from a laboratory in China, perhaps through an accidental leak.

In a statement on May 26, President Biden said he had asked for a report on the origins of Covid-19 after taking office, “including whether it emerged from human contact with an infected animal or from a laboratory accident”.

On receiving it this month, he asked for “additional follow-up”.

President Biden said the majority of the intelligence community had “coalesced” around those two scenarios, but “do not believe there is sufficient information to assess one to be more likely than the other”.

He has now asked agencies to “redouble their efforts to collect and analyze information that could bring us closer to a definitive conclusion”.

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President Biden concluded by saying the US would “keep working with like-minded partners around the world to press China to participate in a full, transparent, evidence-based international investigation”.

Beijing has previously suggested Covid-19 could have come from a US laboratory instead.

In its statement, the Chinese embassy said it supported a full investigation into “some secretive bases and biological laboratories all over the world”.

On May 27, on spokesman for China’s foreign ministry hit out at the “dark history” of the US intelligence community, and said the Biden administration’s “motive and purposes” were clear.

President Biden’s statement came as CNN reported that the president’s administration this spring shut down a state department investigation into whether the virus could have leaked from a Wuhan lab, deeming the probe an ineffective use of resources.

Speculation about the Wuhan Institute of Virology – one of China’s top virus research labs – began in 2020 and was propagated by President Donald Trump.

In April 2020, State Department cables came to light that showed embassy officials were worried about biosecurity at the lab.

The leak allegations were widely dismissed as a fringe conspiracy theory.

Earlier this year, the WHO issued a report written jointly with Chinese scientists on the origins of Covid-19 which said the chances of it having started in a lab were “extremely unlikely”.

The WHO report said the virus had probably jumped from bats to humans via another intermediary animal, but more research was needed.

However, questions have persisted and recent reports attributed to US intelligence sources say three members of the Wuhan Institute of Virology were admitted to hospital in November 2019, several weeks before China acknowledged the first case of the new disease in the community.

Dr. Anthony Fauci, President Biden’s chief medical adviser, has maintained he believes the virus was passed from animals to humans, though he conceded this month he was no longer confident Covid-19 had developed naturally.

President Biden’s statement came the day after Xavier Becerra, US secretary for health and human services, urged the WHO to ensure a “transparent” investigation into the virus’s origins.