“We are encouraging people to remain in their rooms,” she said. “We have the equipment and supplies to take care of them, and people are doing what they need to do.”

Chad Bergevin, who lives opposite the center, said he had learned of the situation in a text message from a neighbor. “It was like, ‘Wow, this is literally less than a football field away from my house,’” he said, adding that he was surprised to see people still seeming to come and go near the center. “I’m sorry, if it were me, I’d have the place on lockdown,” he said.

The indications of a possible spread, and the involvement of a nursing home marked a new, urgent phase in the response to the virus in the United States, where 70 cases have been reported, and until Saturday, none had been fatal. Most of the cases could be explained by overseas travel or contact with someone who had been ill. This week, though, new cases, in California, Oregon and Washington, were the first in the United States in which the cause was mysterious and unknown — a sign, experts warned, that the virus, which has killed more than 2,800 people worldwide and has sickened more than 86,000 others, might now be spreading in the United States.

In the Northwest, especially, health officials were putting in place new precautions given the new cases. They were already discussing the possibility that they might recommend cancellations of public events. They began warning that life in the coming weeks may undergo dramatic change.

By Saturday, 10 people have been treated in Washington State, including the first case of coronavirus that was diagnosed in the United States, a man in his 30s who had traveled in China and has since recovered; several patients were treated at a Spokane area hospital after returning from a cruise ship in Japan; and the first known coronavirus fatality in the United States, a man in his 50s whose death was announced Saturday.

In announcing the death at a news conference, President Trump said the victim was a “wonderful woman” in her 50s, but local officials later said the patient had been a man in his 50s. The Centers for Disease Control and Prevention later said that the patient was, in fact, a man, and that the agency had incorrectly described the patient as a woman.

Few details were known about the man who died, except that he had underlying health conditions and had been a patient at a hospital in Kirkland. He was not known to have traveled abroad, or to have had contact with anyone who had tested positive for the virus, adding to growing signs that the coronavirus may be spreading in the United States. He also had no known connection to the nursing home, officials said.