What are the rules for using incidentally collected information about Americans?

The government puts “raw,” or not yet processed, emails and phone calls it intercepts into repositories that intelligence analysts then query in search of messages relevant to what they are working on. They can search the repositories using keywords or the names of people related to their investigation, including the names of Americans.

When writing surveillance-based reports for broader dissemination within the intelligence community, analysts are supposed to “minimize” any privacy intrusion into Americans. Generally, this requires them to “mask” any names and private information about Americans. For example, a report citing Ivan’s phone call with Joe might recount what Ivan said to “U.S. Person 1,” rather than using Joe’s name and remarks.

When might an American’s name be unmasked in a report?

Minimization rules make an exception to the masking requirement if the American’s words and identity are necessary to understand foreign intelligence. This exception would seem to cover keeping Mr. Flynn’s identity and words unmasked in disseminated materials about his conversation with Mr. Kislyak.

Much remains murky about the separate concerns Mr. Nunes expressed this week about intelligence reports that were unrelated to Russia and contained incidentally collected information about members of the Trump transition team. Mr. Nunes has suggested that he has concerns about unmasked names in them. But the top Democrat on the Intelligence Committee, Representative Adam B. Schiff, has said Mr. Nunes had clarified that they were masked in the reports and the problem was Mr. Nunes could still tell who they were.

Is incidental surveillance a new concern?

No. Long before the present controversies, the government’s handling of Americans’ private communications that its surveillance programs collect incidentally has been the subject of a major public policy dispute.

It’s called the “backdoor search loophole” when officials search raw repositories of surveillance information intending to pull out and read any incidentally collected private messages of an American — especially when those messages were gathered without a warrant in the first place. Notably, while most agencies may only perform such a search for intelligence purposes, the F.B.I. may sometimes do so for ordinary criminal investigations too.

There are two types of warrantless surveillance that incidentally gather Americans’ private communications. Under the FISA Amendments Act, the government, while operating on domestic soil, may target specific foreigners abroad without a warrant. Under Executive Order 12333, the government, while operating abroad, may vacuum up communications in bulk, without targeting anyone.