The hunt for the origins of COVID-19 has gone dark in China, the victim of political infighting after a series of stalled and thwarted attempts to find the source of the virus that killed millions and paralyzed the world for months.

The Chinese government froze meaningful domestic and international efforts to trace the virus from the first weeks of the outbreak, despite statements supporting open scientific inquiry, an Associated Press investigation found. That pattern continues to this day, with labs closed, collaborations shattered, foreign scientists forced out and Chinese researchers barred from leaving the country.

The investigation drew on thousands of pages of undisclosed emails and documents and dozens of interviews that showed the freeze began far earlier than previously known and involved political and scientific infighting in China as much as international finger-pointing.