In addition to what other people already mentioned (how it is illegal), it may depend on jurisdiction. Also whether the country of origin of the source has an extradition treaty with the USA, or if the USA can otherwise (e.g. extrajudicial kidnappings) get the culprit to stand trial in the USA.

EDIT: While it maybe a relatively clear cut case according to US law, other countries (may) have different laws. There are also all kind of potential diplomatic and political obstacles, when this was done by somebody outside of the USA. For instance, good luck if this was a Russian or Chinese citizen.