The closest I can recall is a situation where a gunman tried taking a church hostage (approximately 100 people), was disarmed by an unarmed good Samaritan, and the responding officer shot the good Samaritan (who may have been holding the gunman's gun). https://www.nbcnews.com/news/us-news...m_npd_nn_tw_ma
https://local.nixle.com/alert/6407169/
So, it was not a mass shooting but instead a mass hostage situation.
Good Samaritan survived.