Gov. Janet Mills traveled to California for a “previously scheduled event” Tuesday as federal agents began sweeping immigration operations in Maine, but her Senate campaign team is not sharing more details about the trip.

Mills, a Democrat, flew Tuesday from Boston Logan International Airport to San Francisco, the Bangor Daily News first reported Thursday after obtaining a photo of the governor in a boarding area. The governor’s office referred a Press Herald reporter to a spokesperson for Mills’ U.S. Senate campaign when asked for comment Thursday morning.

Mills “remains in close contact with organizations across Maine to monitor (Immigration and Customs Enforcement) activity and ensure the safety and civil rights of people across Maine are protected,” said Tommy Garcia, the Senate campaign spokesperson.

The U.S. Department of Homeland Security said it launched “Operation Catch of the Day” in Maine on Tuesday, with federal agents targeting about 1,400 people that officials said are “the worst of the worst” criminals in the country illegally. ICE officials said agents made 50 arrests on the first day of the operation, which is focused on the Portland and Lewiston areas. Reporters and bystanders observed masked agents detaining more people Wednesday and Thursday.

But residents without criminal records and with legal asylum claims pending, such as a University of Southern Maine student and a mother of four children, are among those federal agents have detained in the first few days of the operation.

Mills’ campaign did not say when she returned from her trip, but the governor was at Portland City Hall on Thursday morning to take questions from reporters and share her concerns about the initial wave of ICE activity in the state.

Mills and Sullivan oysterman Graham Platner are the chief contenders in the Democratic primary that will decide who faces U.S. Sen. Susan Collins, R-Maine, in November. Platner and his wife said earlier this month they were traveling to Norway for fertility treatments, with Platner calling the cost of in vitro fertilization in the U.S. “astronomical.”

Platner said he is returning to Maine on Friday and will be “in the streets” Saturday to oppose the federal immigration operation.