Health officials in Washington state said Sunday night that a second person had died from the coronavirus.
Researchers said the virus may have been circulating for weeks undetected in the greater Seattle area.
In a statement, Public HealthSeattle & King County said a man in his 70s died Saturday. On Friday, health officials said a man in his 50s died of coronavirus. Both had underlying health conditions, and both were being treated at a hospital in Kirkland, Washington, east of Seattle.
Washington state now has 12 confirmed cases.
State and local authorities stepped up testing for the illness as the number of new cases grew nationwide, with new infections announced in California, Illinois, Rhode Island, New York and Washington state.
Authorities in the Seattle area said two more people had been diagnosed with the COVID-19 virus, both men in their 60s who were in critical condition, and two health care workers in California were also diagnosed.
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A man in his 50s died in Washington on Saturday, and health officials said 50 more people in a nursing facility in Kirkland, Washington, are sick and being tested for the virus.
On Sunday night, the International Association of Fire Fighters said 25 members who responded to calls for help at the nursing facility are being quarantined.
The first U.S. case was a Washington state man who had visited China, where the virus first emerged, but several recent cases in the U.S. have had no known connection to travelers.
In California, two health care workers in the San Francisco Bay area who cared for an earlier coronavirus patient were diagnosed with the virus on Sunday, the Alameda and Solano counties said in a joint statement.
The health care workers are both employed at NorthBay VacaValley Hospital in Vacaville, California, and had exposure to a patient treated there before being transferred to UC Davis Medical Center in Sacramento, the statement said.
That patient was the first person in the U.S. discovered to have contracted the coronavirus with no known overseas travel.
Alameda County declared a state of emergency on Sunday following the news. Elsewhere, authorities announced Sunday a third case in Illinois and Rhode Island and New York's first cases as worried Americans swarmed stores to stock up on basic goods such as bottled water, canned foods and toilet paper.
The hospitalized patient in Rhode Island is a man in his 40s who had traveled to Italy in February. New York confirmed Sunday that a woman in her late 30s contracted the virus while traveling in Iran. The patient is not in serious condition.
She has respiratory symptoms and has been in a controlled situation since arriving in New York, according to a statement from the governor's office.
As the fallout continued, Vice President Mike Pence and Health and Human Services Secretary Alex Azar sought to reassure the American public that the federal government is working to make sure state and local authorities are able to test for the virus.
Both said during a round of TV talk show appearances Sunday that thousands more testing kits had been distributed to state and local officials, with thousands more to come.
They should know we have the best public health system in the world looking out for them, Azar said, adding that additional cases will be reported and the overall risk to Americans is low.
As the cases ticked up, some Americans stocked up on basic supplies particularly in areas with diagnosed cases and began to take note of the impact on daily life.
Stores such as Costco sold out of toilet paper, bottled water and hand sanitizer outside Portland, Oregon, where a case was announced Friday. Sports games and practices were canceled into the coming school week. Some churches said they would not offer communion because of fears of viral spread.
As Americans prepared, researchers at the Fred Hutchinson Cancer Research Center and the University of Washington on Sunday said they had evidence the virus may have been circulating in the state for up to six weeks undetected a finding that, if true, could mean hundreds of undiagnosed cases in the area.
They posted their research online, but it was not published in a scientific journal or reviewed by other scientists.
Trevor Bedford, an associate professor who announced the preliminary findings on the virus in Washington state, said on Twitter late Saturday that genetic similarities between the state's first case on Jan. 20 and a case announced