After reading the article by Glenn Greenwald...

[FarCenter]

29. The preceding paragraph explained it

Under the FAA, which was just renewed last December for another five years, no warrants are needed for the NSA to eavesdrop on a wide array of calls, emails and online chats involving US citizens. Individualized warrants are required only when the target of the surveillance is a US person or the call is entirely domestic. But even under the law, no individualized warrant is needed to listen in on the calls or read the emails of Americans when they communicate with a foreign national whom the NSA has targeted for surveillance.

The key sentence is bolded. If a US person (i.e. a citizen or permanent resident) is inside the US and calls or is called by someone outside of the US, the call can be intercepted without a particular warrant. If a US person is inside the US and calls or is called by a targeted foreign national inside the US, the call can be intercepted under whatever authority is being used to target the foreign national.

For example, suppose that suspected terrorist John Doe's home phone is intercepted by warrant from the FISA court. If Jane Doe uses the phone to call their daughter, the call will be intercepted. In the old days, the person listening to the tap would not record the conversation. These days, I suppose that it gets recorded automatically and gets marked "do not listen to this". If John Doe calls Sam Smith to set up a golf date, the conversation with Sam Smith gets recorded. Therefore, in any wiretapping process, there are people who are unrelated to the investigation that are recorded, either because they use some means of communication that is under surveillance or they are a party to an innocent interaction with the target of surveillance.