Much debate has centered on the question of when the virus arrived in the United States and how early it began to spread among people. Problems and delays slowed the availability of widespread testing for the virus, which has killed more than 40,000 people nationwide.

In January, the authorities identified a series of coronavirus cases from travelers abroad, but they did not identify any community spread of the virus for several weeks.

The federal government had strict rules on who qualified for coronavirus testing, and test kits developed by the C.D.C. — that public health labs began receiving on Feb. 7 — turned out to be faulty. Strict definitions of who could be tested limited what local health officials could do to find out how widespread the virus might be.

“We had to ask the C.D.C. every single time: Does this person meet the case definition? May we send a sample?” Dr. Cody said.

“We had this very, very uncomfortable feeling that we were hearing about a lot of patients who really felt that they were cases but we couldn’t test,” she said.

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Other indications have emerged that the virus may have been spreading earlier than previously known. The Grand Princess cruise ship that departed San Francisco on Feb. 11 had passengers who developed the coronavirus on board. Researchers believe that the virus also began to circulate in the New York area by mid-February. And in early March, researchers found a range of cases with genetic similarities to each other in the Seattle area, suggesting that it had been spreading undetected for weeks.