To add a little bit to bealine's excellent responses so far.
It is absolutely permitted for a passenger to carry items belonging to someone else. HOWEVER, it is a requirement under NASP that items that are identified (via the mandatory security questions or other means) as belonging to "a person other than who placed that baggage in the custody of the operator" (aka "unaccompanied baggage") and subjected to a higher level of screening (Level 3 screening).
Level 3 screening requires that the bag be screened via either hand search, via conventional X-ray at different orientations or via an EDS certified scanner with multi-orientational imaging.
Failure to answer the mandatory security questions accurately however may be a criminal offence and can lead to offloading of baggage (mandatory) denial of travel (somewhat common) and possibly prosecution (very rare).