Chinese scientists refused to share raw data that might bring the world closer to understanding the origins of the coronavirus pandemic, independent investigators for the World Health Organization said Friday.

The investigators, who recently returned from a fact-finding trip to the Chinese city of Wuhan, said disagreements over patient records and other issues were so tense that they sometimes erupted into shouts among the typically mild-mannered scientists on both sides.

China’s continued resistance to revealing information about the early days of the coronavirus outbreak, the scientists say, makes it difficult for them to uncover important clues that could help stop future outbreaks of such dangerous diseases.