Sentence:  Many survivors tried to describe what they felt, but words failed them. Hard to imagine, too, the emotions of the <WSD>first responders</WSD> – the ambulance crews, the medics and the police – faced by ghastly mayhem. It must have felt overwhelming, but they did their jobs and doubtless saved many lives. 
Meaning of the word marked with <WSD> tags:  A member of an emergency service who is first on the scene at an emergency