While it is a crime to attempt to cross into the US in a manner that violates US immigration laws (e.g. “illegal immigration”), it is not illegal to already be within the US without valid immigration papers. Once an “illegal immigrant” makes it across the border they are no longer in violation of any criminal laws simply to be here. At that point they become undocumented immigrants. Deportations and other immigration proceedings are handled by administrative law, not criminal law. Formerly this was handled by the INS and the Bush administration shifted this responsibility to the DHS. It’s really all semantics, it just depends if you use the word “illegal” to mean without permission, or in violation of a criminal statute.