Who shot Magnum?

So I'm watching Lizzie's kidnapping episode, and I've always wondered.
Do they ever find out who killed Magnum?
For some reason I was thinking it was maybe Bishop...?
Not sure if that's just what I'm guessing or he actually did?
I'm not sure why he would, other than to try and look innocent.
(BTW, I've seen all the episodes, so don't worry about any 'spoilers')