As crowds protested the sudden presence of Immigration Customs and Enforcement and other agencies on the streets of Los Angeles, law enforcement officers responded in some cases with tear gas, pepper balls and rubber bullets.

Some protesters reported injuries and accused federal law enforcement of excessive force.

But seeking accountability for a federal agency is challenging.

“These incidents involving federal agents just essentially go into a black hole,” said Michael Gennaco, a former federal civil rights prosecutor who now consults with law enforcement on reforms.