The same person
mm I think you're getting it a bit wrong, no one said the guy Jack saw was on the road or walking near by -.-
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Why does he drop his briefcase?
- Irrelevant
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Has he got only one arm?
Nope
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Was someone killed in the traffic?
no
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He dropped the briefcase, and the gun accidently went off, shooting the wife. Period.
The briefcase is irrelevant to the story
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Was his wife supposed to be with this person at that time, but wasn't because obviously the person was near that road and Jack's wife was nowhere to be seen, and that's how Jack found out?
Nope. Again who said the man was near the road?