Obtaining evidence outside the United States involves considerations unfamiliar to many American prosecutors. Most problems associated with international evidence gathering revolve around the concept of sovereignty. Virtually every nation vests responsibility for enforcing criminal laws in the sovereign. The other nation may regard an effort by an American investigator or prosecutor to investigate a crime or gather evidence within its borders as a violation of sovereignty. Even such seemingly innocuous acts as a telephone call, a letter, or an unauthorized visit to a witness overseas may fall within this stricture. A violation of sovereignty can generate diplomatic protests and result in denial of access to the evidence or even the arrest of the agent or Assistant United States Attorney who acts overseas.
The solution is usually to invoke the aid of the foreign sovereign in obtaining the evidence. The Office of International Affairs advises prosecutors in selecting an appropriate method for requesting assistance from abroad. See USAM 9-13.520 and this Manual at 274 et seq. The method chosen depends on the factors listed in this Manual at 268 to 272.
[cited in USAM 9-13.510]