When it’s a public emergency like a terror attack what happens with the ambulances?

I’m watching patriots day on Netflix and mark whalburg (spell check) was asking for all ambulances to come to the Boston marathon bombing location. In a situation like this are all ambulances from across the city come to the location? What if someone gets shot across town are they doomed or would it still be an ambulance to come get them some help

When it’s a public emergency like a terror attack what happens with the ambulances?
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