Chinese authorities refused to provide World Health Organization investigators with raw, personalized data on early Covid-19 cases that could help them determine how and when the coronavirus first began to spread in China, The Wall Street Journal has reported citing WHO investigators.

The report said there were “heated exchanges” between WHO and Chinese officials over the “lack of detail.”

The Chinese authorities turned down requests to provide such data on 174 cases of Covid-19 that they have identified from the early phase of the outbreak in Wuhan. The investigators are part of a WHO team that this week completed a month-long mission in China aimed at determining the origins of the pandemic and how it spread.

Member states typically provide such data—anonymized, but disaggregated so investigators can see all other relevant details on each case—as part of WHO investigations, the team members pointed out. However, China did not do this.

Chinese officials and scientists provided their own extensive summaries and analysis of data on the cases, the WHO team members told the Wall Street Journal. They also supplied aggregated data and analysis on retrospective searches through medical records in the months before the Wuhan outbreak was identified, saying that they had found no evidence of the virus.

But the WHO team wasn’t allowed to view the raw underlying data on those retrospective studies, which could allow them to conduct their own analysis on how early and how extensively the virus began to spread in China, the team members said.

“They showed us a couple of examples, but that’s not the same as doing all of them, which is standard epidemiological investigation,” said Dominic Dwyer, an Australian microbiologist on the WHO team. “So then, you know, the interpretation of that data becomes more limited from our point of view, although the other side might see it as being quite good.”