This is a good basic article about what to do when faced with traumatic injuries.
A couple of notes to pay attention to. “Get out of Dodge”. The risk of secondary attacks on first responders is high. The noise, the smell, and the visuals that you are presented with will all be distractions that may make you forget this important point.
When I completed my first aid merit badge way back when I was just a young lad of 12 summers, I was stumped on one of the questions the scout master asked. “I have a head wound, you have applied direct pressure but the bleeding won’t stop. What do you do”? I learned about new (to me) pressure points in the neck that day. The futuristic hemostatic clotting agents that are available today are so much more than we had available to us mumble mumble years ago both as a young scout, and later as a young Marine. Make sure you take a look at point four.
No one ever wants to be faced with an event like we saw in Boston but if you are, know what to do and be prepared to do it.