Under US surveillance rules, unmasking an American's identity in a report derived from foreign-intelligence surveillance is routine when necessary to understand (e.g., who was the Russian ambassador talking to?). The NSA did so 10,000 times last year, nearly 17,000 times in 2018.

The same rules apply to intercepts of foreigners talking ABOUT an American. Hypothetical ex: an NSA intercept of Turkish officials discussing how they covertly paid "U.S. Person 1" to lobby for Gulen's extradition & are excited about his growing influence. Who's US Person 1???

Story (w/ my colleague @julianbarnes): GOP senators release list of Obama nat-sec and foreign policy officials who asked to know the identity of masked American(s) in intelligence reports where at least one turned out to be Michael Flynn https://www.nytimes.com/2020/05/13/us/politics/unmasking-flynn.html