Logic Puzzle: Who killed Jeff?

There were four men accused of killing Jeff. Their names were James, Jack, John, and Joe. Each man had to give three statements about what they know.

James says:

John did not kill Jeff.

I never knew Jeff.

I do not know who killed Jeff.

Jack says:

John killed Jeff.

Jeff was my best friend.

James told three lies.

John says:

I did not kill Jeff.

James was lying about knowing who killed Jeff: it was him.

Jeff was my best friend.

Joe says:

Jeff was John’s best friend.

I knew Jeff.

I don’t know who killed Jeff.

Through your investigations, you know that at least one of the men had played a part in killing Jeff, but only one actually killed him. Also, you find out that one man told no lies, one man told one lie, one man told two lies, and the other man told three lies. You do not know who told how many lies, but you are able to use reasoning to find one man of which you can find out that, through logic, is innocent. You ask the innocent man to give you a final statement. You use the number of lies he told to assume whether the next statement he gives you is true or false.

He says:

            "James knew Jeff."

So, using logic, who killed Jeff? Who told how many lies? Which statements are the lies?