ANSWER:

Jeff killed Mike.

How do we know?

Jack is not the murderer, he is the brother of the murderer.

Dan is not the murderer since he ran a marathon, and the murderer recently had his leg amputated.

Ben is not the murderer. He just met Jack. Jack and the murderer grew up together.

We know that Jeff is still alive because he wants to install Ben’s new computer next week.

Mike didn’t grow up with Jack.

If Jack, Dan, Ben, and Jeff are still alive, then Mike is dead.

We can only deduce that Jeff must be the murderer.

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