Dr. Robert R. Redfield, 68, a former military doctor and prominent AIDS researcher who directs the C.D.C., trusted his veteran scientists to create the world’s most precise test for the coronavirus and share it with state laboratories. When flaws in the test became apparent in February, he promised a quick fix, though it took weeks to settle on a solution…

Dr. Stephen Hahn, 60, the commissioner of the Food and Drug Administration, enforced regulations that paradoxically made it tougher for hospitals, private clinics and companies to deploy diagnostic tests in an emergency. Other countries that had mobilized businesses were testing tens of thousands daily, compared with fewer than 100 on average in the United States, frustrating local health officials, lawmakers and desperate Americans.

Alex M. Azar II, who led the Department of Health and Human Services, oversaw the two other agencies and coordinated the government’s public health response to the pandemic. While he grew frustrated as public criticism over the testing issues intensified, he was unable to push either agency to speed up or change course.