The operation began around 6:30 a.m. in the 700 block of 27th Street near West Street. Law enforcement officers were at the scene for hours.
"HSI is conducting a criminal investigation, not a civil immigration or deportation action," Oakland police spokeswoman Officer Johnna Watson said in a statement.
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Oakland is a Sanctuary City, but police Chief Anne E. Kirkpatrick authorized officers to provide traffic control after HSI on Tuesday requested their assistance.
"Chief Kirkpatrick gave strict instructions to the commander and officers that they may not assist HSI with any enforcement efforts," Watson said. "The officers' only role was that of traffic control and neighborhood safety."
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Last month, the Oakland City Council moved to terminate an agreement between the Police Department and ICE allowing some officers to be designated as customs task force officers, but Watson said the department may still "assist our local, state and federal agency agencies when requested as it pertains to traffic control, community safety and officer safety."
James Schwab, a spokesman for ICE in San Francisco, could not confirm what type of investigation was underway or whether any arrests had been made this morning. However, Schwab did explain the relationship between HSI and ICE.
"Under ICE there are two organizations," Schwab said. "One is Enforcement and Removal, that's deportation."
"And then there's Homeland Security Investigations," Schwab said.
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"That's our criminal investigation side."
He added this morning that additional information was unlikely to be released today, but more details might be available Thursday.
"It's an ongoing investigation," Schwab said. "They're going to have to go through everything."
The activist organization By Any Means Necessary is calling for an emergency protest this evening near the scene of the warrant service, 715 27th St. The demonstration begins at 5 p.m. and a march is scheduled for 6 p.m.