There are five people. One of them shot and killed one of the other five. Which man is the murderer?

1. Don ran in the NY City marathon yesterday with one of the innocent men.
2. Mack considered being a farmer before he moved to the city.
3. Jonah is a topnotch computer consultant and wants to install Ben's new computer next week.
4. The murderer had his leg amputated last month.
5. Brad met Jonah for the first time six months ago.
6. Jonah has been in seclusion since the crime.
7. Don used to drink heavily.
8. Brad and Jonah built their last computers together.
9. The murderer is Jonah's brother. They grew up together in Seattle.