Who was the the person that led the walkers in the prison with the vlood when Rick and the gang first were getting settled, I believe it was season two?

It's actually in Season 3, Ep. 4. And it's Andrew, one of the inmates, it is resolved at the end of the episode when Rick, Daryl and Oscar go in the generator room to stop the alarm and get attack sejak Andrew. We thought he died in Ep. 2, when Rick just let him to the walkers. He somehow found a way to survive and took revenge, killing T-Dog, Lori and almost Carol too.

anda can't tell who it is.....