A group of scientists has called for an independent inquiry to consider all possibilities and determine whether the coronavirus came from an animal.
The call comes amid controversy over the investigation organised by the World Health Organisation and China into the origins of Covid-19.
More than 20 signatories said in a letter published by The Wall Street Journal that the existing mission was not independent enough and demanded a new investigation to consider all possibilities.
Half of the joint team are Chinese citizens whose scientific independence may be limited, the letter said.
The criticism comes as the mission considers delaying the release of an interim report, which was expected soon.
The investigators may instead publish a summary statement on the same day as the full report, a WHO spokesman told the Journal.
The organisation expects to have clearer ideas about whether future studies and missions are needed to validate key hypotheses once it has received the full report, and will discuss the next steps with member states, he said.
Last month, the mission rejected speculation that the coronavirus could have leaked from a lab and said instead that it may have jumped to humans through an animal host or frozen wildlife products.
WHO Director General Dr Tedros Adhanom Ghebreyesus later said the agency had not ruled out any hypotheses.
The WHO has faced criticism since the outbreak of the pandemic, with some saying it had been too deferential to China.
Former US president Donald Trump advanced the theory that the virus might have escaped from a high-security virology lab in Wuhan, the Chinese city where the virus was first detected.
The mission followed months of negotiation with China. Stung by criticism that they initially covered up the extent of the crisis, Chinese state media and officials have promoted the theory that the virus did not start in the country, but was brought in.
The scientists who signed the open letter included the laboratory scenario among the possibilities.
They included Steven Quay, chief executive at Atossa Therapeutics, which develops treatments for breast cancer and Covid-19, and Jamie Metzl, a senior fellow at the Atlantic Council.
None of the signatories were members of the WHO-backed mission.