Well, if I saw someone run over by a car, I might scream (rather than say any specific words).  Its hard for me to speculate since i'm not your character...and that's the root of what expression would be made.  What is in keeping with the nature of you character?  Some people would say "oh, no, no no," and just keep repeating words ou of shock, while other people   would hold on to their emotion and call 911.

I'm not an expert on shocking expressions.  I guess I'm not the person to be able to anwer your question. I prefer suspense to shock.

Good luck. I hope someone is able to come up with what you need.