With migrant crossings at the Mexico border at their lowest level in five decades, U.S. Border Patrol agents who normally police the frontier are being sent to cities deep inside the country to assist in federal immigration crackdowns.

The shift has brought agents trained to spot illegal border crossings into densely populated neighborhoods — sometimes with military-style tactics that lead to violent confrontations.

In places such as Chicago, they are joining Immigration and Customs Enforcement and other federal law enforcement officials carrying out U.S. President Donald Trump’s directive to round up people without permission to be in the country.