“I don’t know that we, at least in the United States, have lived this history before,” said Craig Futterman, who directs the Civil Rights and Police Accountability Project at University of Chicago Law School. He added, “This isn’t normal.”

State authorities can and do arrest and prosecute federal agents who violate the law, Mr. Futterman said, although agents who believe they were acting in the line of duty can request immunity in federal court. Investigations into such shootings are sometimes conducted by federal and local agencies in tandem.

More broadly, federal agents have traditionally notified local authorities of most planned operations, both as a matter of courtesy and safety. Agencies often collaborate, splitting up duties according to their strengths — for example, local police are generally more adept at crowd control during protests.

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