A Solano County resident has tested positive for the new coronavirus but had not recently traveled to any foreign country where the virus is spreading and had not had contact with any people with confirmed cases, public health officials said Wednesday.

The case marks the first time that U.S. authorities have not been able to determine the source of a coronavirus infection, and suggests that the virus may be spreading in the community. The patient is being treated at UC Davis Medical Center in Sacramento, UC Davis confirmed late Wednesday evening.

The Centers for Disease Control and Prevention announced the new case of COVID-19 Wednesday afternoon.

“At this time, the patient’s exposure is unknown,” the CDC said in a statement. “It’s possible this could be an instance of community spread of COVID-19, which would be the first time this has happened in the United States.”

CDC spokesman Scott Pauley said the patient was identified “through the public health system,” but he could not say why the person was tested for coronavirus if he or she did not have recent travel in China or a known exposure to an individual with the virus.

Late Wednesday evening, UC Davis publicly released an email that university and hospital officials had sent to employees earlier in the day explaining that the patient arrived at UC Davis Medical Center in Sacramento on Feb. 19 after being transferred from another hospital. The patient was already intubated, on a ventilator and “given droplet protection orders because of an undiagnosed and suspected viral condition,” according to the email, written by David Lubarsky, UC Davis vice chancellor of human health services, and Brad Simmons, interim chief executive of UC Davis Medical Center.

Lubarsky and Simmons wrote in the email that the CDC did not test the patient for coronavirus last week, despite their requests, because the patient did not meet CDC criteria for testing — which includes recent travel to China or exposure to a known case of COVID-19.

The CDC conducted the test Sunday and results came back Wednesday, prompting UC Davis officials to tell “a small number” of hospital employees to stay home and monitor themselves for fever — “out of an abundance of caution,” reads the email. The email was first reported on by the Davis Enterprise and was circulating on social media before the university officially released it.

The CDC said it is possible that the source of the infection will be determined with further interviews with the patient. Public health officials are now attempting to track where the person may have gone in the community and who he or she may have interacted with.

The case is the 60th in the United States, and the 28th in California. The bulk of the infections have been in passengers who were evacuated from a cruise ship in Japan and quarantined at U.S. military facilities, including Travis Air Force Base in Fairfield, in Solano County. The Bay Area county is halfway between San Francisco and Sacramento. It has a population of about 500,000.

Dozens of cruise ship passengers are currently at Travis, and 17 people have been transferred from the air base to Northern California hospitals after testing positive for the virus. Officials with state and local public health were not available Wednesday evening to comment on whether Travis or the hospitals could have been a source of the infection.

Every other person who has tested positive so far had been on the cruise ship, had recently traveled to China, or had close interaction with someone who had recently traveled. But public health officials have warned that community spread of the virus was likely to happen at some point.

“We have been anticipating the potential for such a case in the U.S., and given our close familial, social and business relationships with China, it is not unexpected that the first case in the U.S. would be in California,” said Dr. Sonia Angell, director of the California Department of Public Health, in a statement.

Dr. Art Reingold, an infectious disease expert at UC Berkeley, said that if the case proves to be the first from community spread, “that doesn’t completely surprise me, but it clearly speaks to the fact that this virus is pretty readily transmissible.”

“We’re going to have to see what unfolds over the next days and weeks,” Reingold said. “Whether we start seeing more cases like they had in Italy,” which has reported several hundred cases over the past week or so, “or whether this remains on a low level here, I can’t prognosticate.”

Erin Allday is a San Francisco Chronicle staff writer. Email: eallday@sfchronicle.com