The day-to-day behaviors of undocumented immigrants are significantly affected when local law enforcement officials do the work of federal immigration enforcement. One such behavior, which has been widely discussed in debates over so-called sanctuary policies, is that undocumented immigrants are less likely to report crimes to the police when local law enforcement officials work with U.S. Immigration and Customs Enforcement (ICE) on federal immigration enforcement. However, the mechanism that explains this relationship of decreased trust in law enforcement has not yet been systematically tested. Do undocumented immigrants become less trusting of police officers and sheriffs when local law enforcement officials work with ICE on federal immigration enforcement? To answer this, we embedded an experiment that varied the interior immigration enforcement context in a survey (n = 512) drawn from a probability-based sample of undocumented immigrants. When local law enforcement officials work with ICE on federal immigration enforcement, respondents are statistically significantly less likely to say that they trust that police officers and sheriffs will keep them, their families, and their communities safe; will protect the confidentiality of witnesses to crimes even if they are undocumented; will protect the rights of all people equally, including undocumented immigrants; and will protect undocumented immigrants from abuse or discrimination.