With little local testing throughout February — in part because of botched testing kits from the Centers for Disease Control and Prevention, along with strict guidelines that limited who could get tested — officials were not aware of the virus transmitting locally in the country until Feb. 26, in Solano County, Calif.

Previous cases had involved people who had traveled to China, where the outbreak began, or who had been exposed to someone who was sick. But the Feb. 26 case in Solano County was of unexplained origin. Similar cases of community transmission were quickly identified in nearby Santa Clara County, which includes San Jose, as well as in Washington State and Oregon.

The new test results made public late Tuesday show that even this timeline failed to reveal how long the virus had been circulating. Ms. Dowd had not recently traveled outside the country, the authorities said, and yet she died a full 20 days before the earliest recorded case of community transmission. Another previously unconnected death in Santa Clara County, on Feb. 17, has also now been linked to the coronavirus.

“Each one of those deaths is probably the tip of an iceberg of unknown size,” Dr. Sara Cody, Santa Clara County’s medical officer, said in an interview.