Of the four House representatives believed to be targeted by the tweets (Ocasio-Cortez, Tlaib, Ilhan Omar and Ayanna Pressley), only one isn't a native-born American. Omar emigrated from Somalia to the US in 1992, when she was a child, and became a citizen in 2000.

Twitter didn't comment on why the tweets went unflagged. It's difficult to know whether this was intentional or just a virtue of the flag system not yet being ready. This does, however, underscore the fine line Twitter will have to walk. The company risks opening itself to accusations of bias the moment it labels a politician's tweet, no matter how unambiguous the policy violation might be -- it may feel compelled to tread carefully, especially with some in Congress itching to regulate its behavior.