Los Angles County health officials announced a case of coronavirus with no known source Monday, possibly making it the first instance of community spread of the virus within the county.
Two additional people tested positive for the virus, including the possible case of community transmission.
The city of Long Beach also reported three preliminary cases of COVID-19 on Monday, bringing the total number of confirmed cases countywide to 16.
The three cases in Long Beach must still be confirmed by testing out of the U.S. Centers for Disease Control and Prevention, officials said.
Barbara Ferrer, director of the L.A. County's health department, said one of the two cases confirmed in the county involves a person who has a history of travel to Japan. The country has seen nearly 1,200 cases of the virus, according to Reuters. But the other person's illness lacks a definitive source.
"We cannot at this point identify a known exposure history so we possibly have our first case of community transmission," Ferrer told reporters.
The two patients are in isolation as they face a 14-day quarantine; their "close contacts" will also be quarantined, she said.
According to Ferrer, health authorities are investigating what locations or people they came into contact with.
Long Beach city officials said the three preliminary cases there include two men and one woman — two of whom traveled to a location abroad where the virus was reported. The other person traveled to a location in the U.S. where the virus has spread.
One of the three patients is being hospitalized in stable condition while the other two remain in isolation at home, according to city officials.
Officials with the Long Beach Department of Health and Human Services are investigating what locations they visited or who they may have come into contact with.
"Now that we have three presumptive positive cases, were conducting extensive contact tracing to determine who else may have come into contact with these invididuals, as well as identifying the potential sources of the infection in these cases," Long Beach Health Officer Dr. Anissa Davis said.
Two of those infected were family members who had taken a cruise on the Nile River in Egypt, on which other cases of COVID-19 were reported, Davis said.
The third person had travelled to a place of known community transmission in Northern California, she said.
Ferrer and Davis said health officials are not recommending the closure of schools around the county or other major restrictions.
She said those at highest risk — pregnant women, the elderly and those with underlying health issues — should refrain from attending events with large crowds and practice other forms of "social distancing."
Earlier this month, a woman in Northern California was also diagnosed with coronavirus in what health officials believe was the state's first case of the virus being contracted through community spread.