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Dr. Charles Calisher, a Colorado State University virologist and the lone signatory to completely change his position, told ABC News that he now believes that \"there is too much coincidence\" to ignore the lab-leak theory and that \"it is more likely that it came out of that lab.\"

In addition to the circumstantial evidence pointing to a possible lab leak, allegations that the Chinese government has not been transparent has weighed on the minds of those seeking to examine all possible explanations.

Marion Koopmans, a Dutch virologist who traveled with the WHO to Wuhan for their investigation, said the Chinese government cooperated to an extent, but said \"it was not so easy\" to gather information.

\"Was everything you would wish to see on the table? No,\" she said.

For U.S. government investigators, \"interest spiked considerably in March of 2020 because we were dealing with the very concerning problems of the Chinese government's cover-up of what was happening in Wuhan,\" said Feith, the former State Department official under President Trump.

\"We were concerned that they were not accepting U.S. offers of help that would have involved U.S. scientists and other international scientists getting in on the ground to be able to learn things,\" Feith said. \"We were concerned that the information that the Chinese authorities were giving to the outside world through the press and through the World Health Organization was unreliable-- and might have been deliberately misleading.\"

After the WHO released its March report casting doubt on the lab-leak theory, the organization's chief, Tedros Adhanom Ghebreyesus, said gaps in their probe merited further investigation, and that the team had \"expressed the difficulties they encountered in accessing raw data.\"

\"I do not believe that this assessment was extensive enough,\" Ghebreyesus said. \"Further data and studies will be needed to reach more robust conclusions.\"

\"As I have said,\" he added, \"all hypotheses remain on the table.\"

\"It's important to stay open-minded, because we don't know exactly what happened,\" said Koopmans. \"We are scratching the surface -- and it's important that we learn and collaborate to learn from these outbreaks.\"

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