The FBI has no authority to operate within other nations, so that usually makes the domestic/foreign distinction a moot point. But there is nothing forbidding t

The FBI has no authority to operate within other nations, so that usually makes the domestic/foreign distinction a moot point. But there is nothing forbidding the FBI from operating abroad if the host nation allows for a given investigation. What's more relevant, the FBI has long been involved in domestic investigations of foreigners (exactly as we might use PRISM for, in fact). In fact the FBI technically is the NSA's liaison to Google, Facebook, MS, etc.; NSA has no authority to interface with them directly so they hand it off to FBI, FBI is the one who actually works with the domestic companies.