When entering the U.S. an immigration officer will examine your passport and any other required documents. After this screening, the officer may either pass you onto to customs, which will inspect your luggage. If, however, the first officer has some justification to be suspicious, you will be directed to the secondary inspection. Officers there will examine your passport, visa and other documentation you may have and carefully inspect your luggage.
Immigration and Custom agents are highly trained and are particularly adept at spotting a lie. Questions asked by the immigration officers are designed to see if you are telling the truth. If they determine that you are lying about a material fact, they have the right to detain you. A U.S. citizen has a right to have an attorney present during the secondary inspection, while a noncitizen does not.
At any point during the entry process a person may be detained. Several reasons exist for detention --- being an illegal alien, being designated a national security threat by Homeland Security or having a warrant pending for your arrest. (If the warrant is a local or county warrant, it is less likely that Immigration will detain you.)
If the event the detention is because you are an illegal alien, there are two options: leave the country voluntarily or be ordered by a judge to return to your home country. However, if you have a legitimate claim to enter the U.S. you may choose to contest the deportation. During the time your immigration matter is pending before the administrative law judge, you may be eligible to be released on bond. To get bond or voluntarily return to your home country, you cannot have any outstanding warrants.
If you an alien and do have a warrant or national security hold on you, ICE will detain you in one of its detention facilities. While aliens do not have all rights of U.S. citizens, aliens do get medical care and reasonable jail conditions. (The ICE detention facilities are regularly audited and inspected to ensure this.) If you are an American citizen with any kind of law enforcement hold, you will be turned over to the agency that placed the hold.
