A joint China-World Health Organization (WHO) study into COVID-19 has provided no credible answers about how the pandemic began, and more rigorous investigations are required — with or without Beijing’s involvement, a group of international scientists and researchers said on Wednesday. The joint study, released last week, said the likeliest transmission route for SARS-CoV-2, the virus that causes COVID-19, involved bats and other wildlife. It all but ruled out the possibility it had leaked from a laboratory. In an open letter, 24 scientists and researchers from Europe, the United States, Australia and Japan said the study was tainted by politics. “Their starting point was, let’s have as much compromise as is required to get some minimal cooperation from China,” said Jamie Metzl, senior fellow at the Atlantic Council think tank, who led the drafting process for the letter. The study’s conclusions were based on unpublished Chinese research, while critical records and biological samples “remain inaccessible,” the letter said.