Troubling new signs that the coronavirus is spreading in the US emerged on Friday, as cases not explained by overseas travel or contact with a person known to be infected were reported in California, Oregon and Washington state.

Officials from the three states announced that their testing had found new cases: a high school student from north of Seattle; an employee of an elementary school in Oregon, near Portland; and a woman in Santa Clara county, Calif., in the heart of Silicon Valley.

Sixty-five cases of the virus have been reported in the US, but until this week, all the cases could be explained by overseas travel or contact with someone who had been ill. The three new cases on Friday, and a case earlier in the week, in California, were the first in the US where the cause was mysterious and unknown — a sign, experts warned, that the virus, which has killed more than 2,800 people worldwide and sickened tens of thousands of others, might now be spreading in this country.

“If we were worried yesterday, we are even more worried today,” said Dr William Schaffner, an infectious disease specialist at Vanderbilt University Medical Center. “Now we have to ask: How widely, really widely, is this virus out there?”

As word emerged of the unexplained cases, local officials scrambled to trace everyone who had come in contact with those who were ill. California health officials said they were increasing testing. And in Washington state, officials suggested that people needed to prepare for the possibility of schools closing and businesses keeping workers home.

“We’re going to be increasingly recommending that people try and avoid crowds and close contact with other people,” Dr Jeff Duchin, health officer for Public Health Seattle & King County, said. “We may get to a point where we want to recommend cancelling large public gatherings — social events, sporting events, entertainment — until we get over a hump of what might be a large outbreak.”

In some new cases, officials described situations where those who were ill may have had contact with many people in recent days.

The student in Washington state who tested positive for coronavirus, according to state officials, had visited two health clinics and gone to his school in Snohomish county.