From City Journal:

Some of the most violent criminals at large today are illegal aliens. Yet in cities where the crime these aliens commit is highest, the police cannot use the most obvious tool to apprehend them: their immigration status. In Los Angeles, for example, dozens of members of a ruthless Salvadoran prison gang have sneaked back into town after having been deported for such crimes as murder, assault with a deadly weapon, and drug trafficking. Police officers know who they are and know that their mere presence in the country is a felony. Yet should a cop arrest an illegal gangbanger for felonious reentry, it is he who will be treated as a criminal, for violating the LAPD's rule against enforcing immigration law.

The LAPD's ban on immigration enforcement mirrors bans in immigrant-saturated cities around the country, from New York and Chicago to San Diego, Austin, and Houston. These "sanctuary policies" generally prohibit city employees, including the cops, from reporting immigration violations to federal authorities...

...In Los Angeles, 95 percent of all outstanding warrants for homicide (which total 1,200 to 1,500) target illegal aliens. Up to two-thirds of all fugitive felony warrants (17,000) are for illegal aliens...

[much snipped for brevity]