After weeks of frustration, the researchers started testing the samples anyway in late February — and one swiftly came back positive. That case — a local teenager with no recent travel history — showed that the virus had already established itself on American soil without anybody knowing it.

“It must have been here this entire time,” Dr. Helen Y. Chu, an infectious disease expert in Seattle, recalled thinking. “It’s just everywhere already.”

Federal officials had said no because the researchers’ lab was not certified for clinical work and the test subjects had not given permission, among other reasons. On Monday night, they ordered the researchers to stop testing their samples.

Hear more: Ms. Fink, who covers global public health for The Times, discusses the story, and the wider consequences of lags and lapses in testing in the U.S., on today’s episode of “The Daily” podcast.

A scramble for action in Washington, D.C.

“We have got to assume it is going to get worse and worse and worse.”

That was Dr. Anthony S. Fauci, the director of the National Institute of Allergy and Infectious Diseases, warning Congress that the coronavirus would keep spreading rapidly in the United States, which crossed the 1,000-case threshold overnight.

President Trump said he would address the nation on the coronavirus crisis at 9 p.m. Eastern.

Senate and House leaders were racing on Wednesday to put together a bipartisan short-term bill that could be sent to Mr. Trump this week. It would most likely focus on paid leave, enhanced unemployment insurance, food assistance and help for small businesses affected by the outbreak.

Worried about your taxes? The Internal Revenue Service said it was thinking about extending the April 15 deadline for most people to pay their 2019 income tax. (Filing extensions are easy to get, but you usually still have to pay on time, even if you file late.)