QuietAndEffective, Immigration is a federal function.  Immigration enforcement is the function of the federal Immigration and Customs Enforcement agency (ICE), and it is optional for local police departments to cooperate with ICE.  HPD officers would not be breaking their Code of Conduct if they don't cooperate with ICE.  

You may not know that ICE engages in a practice that has been determined by several federal judges to be unconstitutional.  ICE issues administrative detainers that are used to hold people in jail who have been arrested -- for 48 hours longer than their release date.  These are NOT warrants signed by a judge that comply with constitutional due process rights that all people on U.S. soil have, including undocumented immigrants.  Several federal judges have ruled that ICE administrative detainers are not only illegal because they exceed ICE's authority, but also unconstitutional.  Local law enforcement agencies should not honor ICE administrative detainers.