Federal officials confirmed the number of U.S. cases of 2019 novel coronavirus had reached 11 on Monday, describing the increasingly deadly global crisis as “explosive” and “unprecedented,” and suggesting it amounted to a potential pandemic.

Two people over the weekend in California became the second domestic case of person-to-person transmission of the virus, said Nancy Messonnier, director of the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention. She painted a picture of a dogged but realist federal and state approach to containment that now included measures in place at 11 airports across the U.S.

“We expect to see more cases of person-to-person spread among close contacts,” she told reporters, later adding, “The goal here is to slow entry of this virus into the United States.”

According to Messonnier, 167 people under investigation for the virus in the U.S. had tested negative for infection, and officials had tests on 82 people pending as of Monday morning. She said the CDC had isolated the virus and planned to submit materials to the Food and Drug Administration to facilitate the dispersal of potentially life-saving medical products that might help facilities be better able to identify cases without transmitting samples to CDC headquarters in Atlanta, as they have been.

California has now become a domestic epicenter of infections, with two infections confirmed in Santa Clara County, in the Bay Area; two more in San Benito County, near Monterey, where a man appeared to infect his wife; and, previously, one case each in Orange County and Los Angeles County, in Southern California.

On Saturday, a UMass-Boston college student returning from Wuhan was also confirmed to be infected. Initial U.S. cases were identified in suburban Seattle, in Chicago (where a woman was believed to have infected her husband upon returning from abroad), and in an individual affiliated with Arizona State University in Tempe.

Worldwide, the novel coronavirus has been linked to at least 361 deaths in China and least one in the Philippines. It had infected at least 17,000 people in China as of Sunday, according to The New York Times, which suggested both that its rate of infection was higher than the 2002-03 SARS epidemic and that its mortality rate appeared to be significantly lower.

On the call with reporters, CDC officials also offered more details on a travel ban imposed by way of a presidential proclamation Friday. New airports covered by the federal response include Dulles in Virginia, Newark Liberty in New Jersey, Dallas-Fort Worth in Texas, and Detroit Metropolitan. Foreign nationals who have visited China in the last 14 days will be barred from entering the country, with some exemptions. Both citizens and “exempted persons” will see additional health assessments and, if symptomatic, transferred and potentially quarantined for 14 days.

Just how easily the disease might spread from infected people within the United States remains to be seen. While some studies and reports have suggested asymptomatic people can spread the illness, the CDC at one point pushed back on that conclusion, before nodding to growing evidence of the possibility on Friday.

Then again, some people might be higher risks of spreading infection than others.

“There has been a lot of discussion among clinicians and scientists about what is something called the R0, which is how many people on average does one person infect,” Davidson Hamer, a professor of global health and medicine at Boston University, told The Daily Beast. “The estimates are somewhere between 1.5 and 3, but again those are just estimates.”

“In SARS, there were what we call ‘super spreaders,’ where one person could infect or appear to infect many, many people,” he added. “We don’t understand how that occurred. We don’t know if there are super spreaders with this novel coronavirus yet.”

—Susan Zalkind contributed reporting to this story.