There are five people: Dan, Mike, Jeff, Jack and Ben.
Statement 9 tells that Jack is not murderer.
Statement 6 tells that Jack is not killed.
Statement 1&4 tells that Dan is not murderer because he cannot run for marathon if he is the murderer with his legs amputated. But we don't know if Dan is survived or not.
Statement 7 is useless as Dan is not murderer
Statement 2 & 9 tells that Mike is not the murderer as he is first living in rural areas but the murderer grows up in Seattle which is a city.
Statement 5 & 9 tells that Ben is not the murderer as he met Jack for the first time six months before so they are not brothers.
Therefore, Jeff is the murderer.
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