Homeland Security Investigations, a branch within Immigration and Customs Enforcement, focuses on combating cross-border criminal activity and is billed as the investigative arm of the Department of Homeland Security. While many of its investigations involve immigrants, including some in the country illegally, many do not.
But its connection to ICE - at a time when the agency is under fire because of its role in deportations and enforcing President Donald Trump's aggressive immigration policy - has caused friction with some law enforcement agencies.
In California, where a "sanctuary" law, Senate Bill 54, went into effect last year to provide protection for immigrants in the country illegally, agents have voiced frustration over police departments pulling out from operations - sometimes at the last moment - as well as withdrawing from task forces and slowing down investigations.
"Now all of a sudden I have an agent in the street saying, 'This police department doesn't want to work with us because my cred says ICE,'" said Joseph Macias, special agent in charge for Homeland Security Investigations Los Angeles.