According to Newsom's tweet, the family learned late Friday evening that they had come into contact with a California Highway Patrol officer who had tested positive for COVID-19. The officer provides security for the Newsom family.
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Newsom and his wife, Jennifer, did not have contact with that officer, but three of the four children did. Newsom's children range in age from 4 to 11.
Newsom says he and his family tested negative for the virus on Sunday and are following "local guidance" by quarantining for 14 days.
RELATED: California Gov. Gavin Newsom's child quarantined for possible virus exposure
On Friday, a spokesman for the governor said one of his children may have been exposed to COVID-19 at school.
The child began a 14-day quarantine "from the date of exposure" after the family was told a classmate at the private school had tested positive for COVID-19, a spokesperson for the governor, Nathan Click, said in an emailed statement.
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The school where Newsom's child may have been exposed will stop in-person learning and move to remote learning after the Thanksgiving break through the second week of December, in part because of the higher infection risks associated with holiday activities, according to an email Politico said it had obtained.
The family is quarantining at their home in Sacramento County. They will be tested regularly, spokesman Jesse Melgar said in an emailed statement to the Associated Press.
Newsom said last month that his children had returned to their private school in Sacramento, sparking criticism even as millions of public schoolchildren continue to study online or through other distance learning measures because of COVID-19 concerns.
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RELATED: Gov. Newsom says he shouldn't have attended birthday party at French Laundry amid COVID-19 surge
The governor also drew fire this week for apparently flouting the very social distancing rules that he has urged Californians to follow assiduously to curb a spiking COVID-19 infection rate.
Newsom said this week he made a "bad mistake" by attending a friend's birthday dinner on Nov. 6 at the ritzy French Laundry restaurant in Napa County. He apologized and described the dinner as outdoors. But photos obtained by the Fox 11 TV station in Los Angeles show Newsom and his wife, Jennifer Siebel Newsom, sitting maskless around a crowded table of 12 - including several lobbyists - inside a room that was enclosed on three sides.
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The Associated Press contributed to this report.