We assess that a Shia homegrown violent extremist (HVE) attack in the US is highly unlikely absent a catalyzing event that could galvanize some US-based Shia to engage independently in violence. Given sustained bilateral US-Iran tensions, the occurrence of such a catalyst could prompt Shia HVE activity relatively quickly, underscoring the benefits of early engagement with Shia communities about indicators of HVE radicalization. Potential triggering events for such Shia HVE violence include US military action against Iran and Lebanese Hizballah, Shia leadership or senior clerics sanctioning violence in the US, prominent Sunni government attacks on Shia, or high-profile anti-Shia activity in the US, judging from the results of a structured NCTC brainstorming exercise.