“Yes, people will in this country be able to find out, but not today,” said Eva Harris, a professor of infectious diseases at the University of California, Berkeley, who is studying exposure to the virus over time in the Bay Area.

Deaths from January and February are also getting new scrutiny after an announcement this week in Santa Clara County, Calif., that a woman who died on Feb. 6 had been found to have the coronavirus. Her death occurred weeks earlier than what had previously been thought to be the first death in the United States from the virus.

Dr. Michelle Jorden, the Santa Clara County Medical Examiner-Coroner, said her office was investigating other deaths as well. Her office has sent samples of suspicious cases to the Centers for Disease Control and Prevention, and some are still pending.

Gov. Gavin Newsom of California has said investigators were looking at coroner and autopsy reports going back to December in some of the state’s counties to determine if there might be other, earlier deaths caused by the coronavirus.

Experts said it would be difficult to distinguish, in hindsight, between the seasonal flu and the coronavirus.

Dr. Jeffrey V. Smith, the Santa Clara county executive and a medical doctor, said his wife, also a doctor, reported being puzzled by patients she was seeing in the Bay Area in December.

“I remember her telling me back in December of a number of patients who came in with flulike symptoms who were testing negative for the flu,” Dr. Smith said. “I just wonder if those were patients that had coronavirus.”