The World Health Organization team investigating the origins of coronavirus has visited a Wuhan laboratory that has become the focus of theories suggesting a leak was responsible for the pandemic.

The heavily-guarded team of experts was escorted from the cordoned-off wing of their hotel to the Wuhan Institute of Virology on Wednesday by a delegation that included plainclothes security as well as foreign ministry staff.

Peter Daszak, a member of the team and head of EcoHealth Alliance, a non-governmental organisation, told journalists the WHO experts were “asking all the questions that need to be asked”.

The team of epidemiologists, zoologists, virologists and public health experts has been tasked with resolving the politically charged question of how Sars-Cov-2, the virus that causes Covid-19, first infected humans.

Many China critics, including Mike Pompeo, the former US secretary of state, have argued that the Wuhan outbreak may have resulted from an accidental leak during experiments on bat coronaviruses at the Wuhan Institute of Virology. The institute and Beijing have fiercely denied the allegation.

Apart from evidence of a genetically similar strain of coronavirus in bats, many questions about the origins of the virus remain unanswered, including whether an intermediate host species was involved.

Since completing a 14-day quarantine in a Wuhan hotel last Thursday, the team has visited the Huanan seafood market connected to the first known cluster of infections as well as hospitals that treated early cases, an animal disease research facility and the provincial Centre for Disease Control.

Critics have raised fears that the long delay before the WHO gained access to these sites would make it difficult to unearth conclusive evidence during the two weeks of field visits.

For example, local authorities dismantled and sanitised the Huanan market early last year. A visit to a propaganda exhibit extolling Wuhan’s response to the outbreak drew accusations that the WHO investigators were being excessively deferential to Beijing.

CGTN, the international arm of China’s state broadcaster, has rejected accusations that the investigation was stage-managed. It said the destinations on the itinerary were requested by the WHO and the lack of media access during the visits was to avoid “distractions”.

But one state media employee told the Financial Times that senior editors instructed journalists not to interview the WHO experts without Chinese foreign ministry approval.

The WHO has stressed that the probe is primarily concerned with gathering information to prevent outbreaks. Yet the investigators also face the difficult task of sorting fact from fiction among a range of origin theories.

Chinese officials have suggested that the Wuhan outbreak was seeded by the virus brought to the city on frozen foods or packaging. International virologists think that is improbable.

The Biden administration has avoided criticising the WHO or endorsing any particular origin theory but called last week for a “robust and clear” investigation.

Antony Blinken, US secretary of state, told MSNBC that China was “falling far short of the mark” in providing access to international experts. “That lack of transparency, that lack of being forthcoming, is a profound problem and it’s one that continues,” he said.

Additional reporting by Nian Liu in Wuhan