The coronavirus may have been circulating for weeks undetected in Washington state, a preliminary finding that could mean hundreds of undiagnosed cases in the state that’s also home to the nation’s first confirmed infection and now the first death, researchers said Sunday after analyzing genetic samples of the pathogens.

State and local authorities stepped up testing for the illness Sunday as the number of new cases grew nationwide, with new infections announced in Illinois, Rhode Island and Washington state. Authorities in the Seattle area said two more people had been diagnosed with the coronavirus, both men in their 60s who were in critical condition. Those cases brought the numbers in the Seattle area to six. Fifty more people in a nursing facility in Kirkland, Washington, are sick and being tested for the virus.

Elsewhere, authorities announced on Sunday a third case in Illinois and Rhode Island’s first case 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 earlier in February.

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 coronavirus. Both said during a round of TV talk show appearances Sunday that thousands more kits to test for coronavirus 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 Americans prepared, researchers at the Fred Hutchinson Cancer Research Center and the University of Washington on Sunday said they had evidence that the coronavirus 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. The research was not published in a scientific journal or reviewed by other scientists.

Trevor Bedford, an associate professor at the Fred Hutchinson Cancer Research Center and the University of Washington, said on Twitter late Saturday that genetic similarities between the state's first case on Jan. 20 and a case announced Friday indicated the newer case may have descended from the earlier one. The Jan. 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.

Bedford did not immediately reply to an e-mail requesting an interview Sunday.

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 associated professor of epidemiology at the Johns Hopkins Bloomberg School of Public Health.

"And that was what a lot of us was thinking was likely."

Pence, named by the president to be the point-person overseeing the government's response, said more than 15,000 virus testing kits had been released over the weekend. And, the administration is working with a commercial provider to distribute 50,000 more, he said.

The vice president said testing was among the first issues raised by governors he's spoken with so far.

"We're leaning into it," Pence said.

Azar said more than 3,600 people already have been tested for coronavirus and the capability exists to test 75,000 people. He forecast a "radical expansion of that" in the coming weeks.

Pence and Azar spoke a day after President Donald Trump approved new restrictions on international travel to prevent the spread within the U.S. of the new virus, which originated in China. There are now more than 80,000 cases worldwide and nearly 3,000 deaths.

The new U.S. travel restrictions apply to Iran, although travel there by Americans already is severely limited, as well as heavily affected regions of Italy and South Korea. Trump tweeted Sunday that any travelers from those countries will be screened when they arrive in the U.S.

The number of known coronavirus cases in the U.S. had reached 70 as of Sunday.

Trump said Saturday at a White House news conference that he was thinking about closing the southern border with Mexico as a precaution. Azar said Sunday that Mexico has few coronavirus cases and that it would take a dramatic change in the circumstances there to prompt serious consideration of a border shutdown.

The president, Azar said, "was trying to say everything's on the table."

"We will take whatever measures are appropriate and necessary to protect the American people, but we don't forecast doing that any time soon," he said of closing the border.

Pence noted that an infectious disease expert is joining an existing White House coronavirus task force on Monday. Last week, Pence announced the addition of Debbie Birx, a State Department ambassador-at-large and medical doctor who is the administration's global HIV/AIDS response coordinator, to the virus panel.

Despite calls by Trump and Pence for political unity in the face of the viral threat, the issue has become mired in the partisan rancor in Washington, with both Republicans and Democrats accusing each other of mining the issue for political gain.

Trump, at a political rally last week, accused Democrats of "politicizing" the issue and said their criticism of his handling of the public health challenge was their new "hoax."

At the White House on Saturday, Trump said he was not trying to minimize the threat from the virus.

"Again, the hoax was used in respect to Democrats and what they were saying," he said Saturday.

Democratic presidential candidate Joe Biden, who emerged victorious Saturday night from South Carolina's primary, criticized the administration over the availability of testing kits.

Biden also panned the administration's decision to have political appointees Pence and Azar, neither of whom are scientists by training, appear on the Sunday shows, instead of an expert like Dr. Anthony Fauci, the National Institute's of Health infectious disease chief.

Biden claimed the administration doesn't have testing kits. Pence and Azar said thousands of kits had been distributed.

Azar said he didn't know what Biden was talking about when the former vice president said testing kits didn't exist. Azar said the Center for Disease Control and Prevention had developed a lab test for coronavirus with "historic speed."

Pence was interviewed on CNN’s “State of the Union” and Azar commented on “Fox News Sunday,” CBS’ “Face the Nation” and ABC’s “This Week.” Biden commented on CNN.