Health officials in Washington state said on Sunday that a second person had died from the coronavirus. The man, in his 70s, was in a nursing facility near Seattle, where many others have tested positive for the coronavirus. 

On Friday, health officials announced the first U.S. death from coronavirus. Both men had underlying health conditions, and both were being treated at a hospital in Kirkland, Washington, east of Seattle.

The first U.S. case was a Washington state man who had visited China, but several recent cases in the U.S. have had no known connection to travelers. Researchers said that the virus may have been circulating for weeks undetected in Washington state.

There are now 12 confirmed cases in Washington state. 

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 January 20 and a case announced Friday indicated the newer case may have descended from the earlier one. The January 20 case was the first known case in the U.S.

"I believe we're facing an already substantial outbreak in Washington State that was not detected until now due to narrow case definition requiring direct travel to China," he said on Twitter.

Scientists not affiliated with the research said the results did not necessarily surprise them and pointed out that for many people – especially younger, healthier ones – the symptoms are not much worse than a flu or bad cold.

"We think that this has a pretty high rate of mild symptoms and can be asymptomatic. The symptoms are pretty non-specific and testing criteria has been pretty strict, so those combinations of factors means that it easily could have been circulating for a bit without us knowing," said Justin Lessler, an associate professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

Dr. Adam Lauring of University of Michigan called the findings "high quality work" from scientists who've done similar work with the flu virus for years.

"They show their data and they show their work," Lauring said. "It's more than a series of tweets" because the researchers back up what they found with data that they've shared online. "If there's something wrong, someone will find it."

Dr. Carlos del Rio of Emory University School of Medicine said the findings are from respected researchers in genomic sequencing and they make sense because of the geographic proximity of the two cases.

"This is a good time to reinforce the things we all should be doing to stop the spread of flu. Wash your hands. Don't touch your face. If you have a cold, stay home [...] It's a good time to remind ourselves of that," he said.

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.

Authorities in the Seattle area reported four new cases of coronavirus on Sunday, including the man who died. Two health care workers in California were also diagnosed. Of the new Washington state cases, two were women, one in her 80s and another in her 90s. Both were in critical condition. A man in his 70s was also in critical condition. All three were from the LifeCare nursing facility in Kirkland, Washington, where health officials said 50 people are sick and being tested for the virus.

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, and New York.

The number of known coronavirus cases in the U.S. had reached at least 80 as of Sunday, counting people evacuated from a cruise ship and the city of Wuhan in China.

Trump said Saturday at a White House news conference that he was thinking about closing the southern border with Mexico as a precaution.

(With input from AP)