The head of the World Health Organization said it was “premature” to rule out the possibility that the novel coronavirus leaked from a laboratory, in comments to reporters on Thursday.

The WHO is “asking actually China to be transparent, open and cooperate, especially on the information, raw data that we asked for at the early days of the pandemic,” director-general Tedros Adhanom Ghebreyesus said at a press conference. Tedros said there had been a “premature push” to rule out a lab leak as the origin of the coronavirus pandemic.

“I was a lab technician myself, I'm an immunologist, and I have worked in the lab, and lab accidents happen,” Tedros said.

The hypothesis that the coronavirus leaked from a lab in Wuhan, China, has received renewed scrutiny in recent months, with President Biden ordering the U.S. intelligence community to assess the likelihood of a leak. Wuhan is home to three laboratories that study bat coronaviruses, including the high-security Wuhan Institute of Virology.

An investigation initiated by the WHO concluded in March that a lab leak was likely not responsible for the pandemic. However, even after that report, Tedros said at the time that “all hypotheses remain on the table” pertaining to the origin of the pandemic.

“Although the team has concluded that a laboratory leak is the least likely hypothesis, this requires further investigation, potentially with additional missions involving specialist experts, which I am ready to deploy,” Tedros said.

China has refused to allow independent investigators access to data from the Wuhan Institute of Virology from early in the pandemic.

Former Centers for Disease Control and Prevention head Robert Redfield has alleged that the WHO was too “compromised” to conduct an adequate investigation into the coronavirus’ origins in the early days of the pandemic.

“I think they were highly compromised,” Redfield told Fox News last month. “Clearly, they were incapable of compelling China to adhere to the treaty agreements that they have on global health, because they didn't do that. Clearly, they allowed China to define the group of scientists that could come and investigate.”