Getting arrested can bring a world of trouble, and getting arrested without legal permission to be in the country can make the situation worse. Because different jurisdictions have their own procedures for processing arrests and operate under different laws, what actually happens to an undocumented immigrant in such a situation varies widely.
An arrest is usually processed in the following manner: The arrestee will be removed to a station house or holding facility for processing, at which time he will be identified, his fingerprints will be taken and the arresting officer will complete his necessary paperwork. Failure to identify oneself, or a lack of photo ID on the person of the arrestee, could arouse suspicion about his legal status. However, if an undocumented person has a valid ID unrelated to his citizenship status, he could manage to go through the arrest process without his immigration status becoming an issue.
Immigration law is the purview of the federal government. Controversial laws have passed in Arizona and other states, however, empowering law enforcement officers to make arrests and inquire about citizenship status based on suspected immigration law violations. Such laws present numerous constitutional issues and, as of this writing, are being challenged in court. State and local police forces, therefore, generally do not enforce immigration law and do not get involved in questioning an arrestee about her immigration status. However, if it is revealed that a person is undocumented, she is turned over to Immigration and Customs Enforcement (ICE).
Illegal immigrants still retain Fourth Amendment protections against unreasonable search and seizure and Fifth Amendment protections against self-incrimination. Arrestees must provide their names (in accordance with the Supreme Court's 2005 decision in Hiibel v. Nevada), but they also have a right not to speak and the right to an attorney. These rights apply when in police custody as well as in the custody of ICE.
ICE enforces immigration law at the level of individual suspects. Arrestees determined to be in the country illegally are handed over to ICE officials, who will remove them to an immigration detention facility. They are held there until they can see an immigration judge, which can take up to two weeks, but some arrestees may be released on bond. ICE determines those cases that are eligible for bond. Immigration proceedings are conducted separately from criminal proceedings related to the arrest, though a criminal conviction could affect immigration proceedings, which may lead to deportation.