8 U.S.C. § 1324 : US Code - Section 1324 - Bringing in and harboring certain aliens
http://codes.lp.findlaw.com/uscode/8/12/II/VIII/1324
Encouraging and Harboring Illegal Aliens
It is a violation of law for any person to conceal, harbor, or shield from detection in any place, including any building or means of transportation, any alien who is in the United States in violation of law.
HARBORING MEANS ANY CONDUCT THAT TENDS TO SUBSTANTIALLY FACILITATE AN ALIEN TO REMAIN IN THE U.S. ILLEGALLY.
The sheltering need not be clandestine, and harboring covers aliens arrested outdoors, as well as in a building.
The penalty for felony harboring is a fine and imprisonment for up to five years for each alien harbored.
The penalty for felony alien smuggling is a fine and up to ten years' imprisonment.
Where the crime causes serious bodily injury or places the life of any person in jeopardy, the penalty is a fine and up to twenty years' imprisonment.
If the criminal smuggling or harboring results in the death of any person, the penalty can include life imprisonment.
Convictions for aiding, abetting, or conspiracy to commit alien smuggling or harboring, carry the same penalties.