In response, Chinese officials and state news media have stepped up their criticism of American officials’ comments.

Only days before Mr. Zhao’s latest post, the Xinhua news agency published a commentary denouncing “Washington’s poisonous coronavirus politics” and warning that spreading rumors simply encouraged “fear, division and hate.”

“Their dangerously irresponsible statements are highly counterproductive at this drastic hour that demands solidarity and cooperation,” the commentary, written by Gao Wencheng, said, “and could be much more menacing than the virus itself.”

The coronavirus, according to all evidence, emanated from Wuhan, China, in late December. Scientists have not yet identified a “patient zero” or a precise source of the virus, though preliminary studies have linked it to a virus in bats that passed through another mammal before infecting humans.

A senior official of China’s National Health Commission, Liang Wannian, said at a briefing in Beijing last month that the likely carrier was a pangolin, an endangered species that is trafficked almost exclusively to China for its meat and for its scales, which are prized for use in traditional medicine.

The first cluster of patients was reported at the Huanan Seafood Wholesale Market, and studies have since suggested that the virus could have been introduced there by someone already infected. Wuhan and the surrounding province of Hubei account for the overwhelming amount of cases and deaths, so there is no scientific reason to believe the virus began elsewhere.

Mr. Zhao’s assertion began with a post linking to a video of the director of the U.S. Centers for Disease Control and Prevention, Robert R. Redfield, testifying before the House on Wednesday and suggesting that some flu deaths might have been caused by the coronavirus.