My neighbor's nanny is from Mexico, and I strongly suspect she doesn't have her papers. Sometimes I give her a ride home, because she lives near my gym. But someone told me that I might be committing a crime. Is that true? What type of risk am I taking here?
A federal law does exist that makes it a crime to transport or attempt to transport a noncitizen within the U.S., but it's meant to cover narrower situations than the one you are describing. The law is found within the Immigration and Nationality Act (I.N.A.), at Section 274(a)(1)(A)(ii).
In order to convict someone under this section of the law, the prosecutors would need to show that:
Although you have clearly been driving (transporting) someone around, it sounds like you aren't even sure of her immigration status. In fact, there's no reason to jump to conclusions. Even if the nanny doesn't have a green card (lawful permanent residence), it's possible that she has at least temporary permission to be in the U.S., along with a work permit, perhaps as an applicant for asylum or by some other legal means. Government backlogs are so long that some people wait years for final decisions on immigration applications, and can in some cases work while they wait.
More importantly, nothing you've described sounds like you were attempting to "further" the person's legal violation (if one exists). The classic situation this law is meant to address is that of a driver who picks up unlawful entrants right after they've crossed the U.S. border and takes them closer to civilization, likely motivated by profit.
By contrast, not even the act of transporting undocumented workers from one job site to another is considered a crime under this section. (See U.S. v. Moreno, 561 F.2d 1321 (9th Cir. 1977). Similarly, the law is carefully written so as to avoid the possibility that a bus or taxi driver could be prosecuted for unknowingly picking up an undocumented immigrant.
If, despite this description, you're still worried that the worst might happen, you probably want to know the possible penalty for this crime. Penalties can include a fine, a prison term of up to five years, or both. That penalty goes up to a possible ten years if the driver was acting for commercial advantage or private gain, and to 20 years to life in prison if anyone is seriously injured or dies as a result of the crime.
Consult a lawyer if you feel you need more information or a personal analysis.