From the Document: "Criminal law is usually territorial. It is a matter of the law of the place where it occurs. Nevertheless, a number of American criminal laws apply extraterritorially outside of the United States. Application is generally a question of legislative intent, express or implied. [...] Although the crimes over which the United States has extraterritorial jurisdiction may be many, so are the obstacles to their enforcement. For both practical and diplomatic reasons, criminal investigations within another country require the acquiescence, consent, or preferably the assistance, of the authorities of the host country. The United States has mutual legal assistance treaties with several countries designed to formalize such cooperative law enforcement assistance. It has agreements for the same purpose in many other instances. Cooperation, however, may introduce new obstacles. Searches and interrogations carried out jointly with foreign officials, certainly if they involve Americans, must be conducted within the confines of the Fourth and Fifth Amendments. And the Sixth Amendment imposes limits upon the use in American criminal trials of depositions taken abroad. [...] This report is available in an abridged version, stripped of its attachments, bibliography, footnotes, and most of its citations to authority, as CRS [Congressional Research Service] Report RS22497, Extraterritorial Application of American Criminal Law: An Abbreviated Sketch, by Charles Doyle."
CRS Report for Congress, 94-166
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html