Looking at two minute intervals across all verified tweets, it appears that detecting mass shootings quickly via social media is possible with some specific filtering. This is looking at all tweets over the past week where "breaking" and "shooting" were in the text.
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The huge spike is when a majority of news accounts started to tweet about the Virginia Beach shooting. There is a smaller spike before the main spike where some accounts took more of a risk on unconfirmed information (but were correct in this situation).