POSTED BY STUART LEVINE

Here's the biggest mystery the "CSI" team will have to uncover: What's up with the new guy?

CBS entertainment topper Nina Tassler dished out a few hints about the character who will be taking over the team when William Petersen leaves the show.

"It's about an outsider coming in. He comes in with a genetic profile the same as a serial killer. He knows this about himself and it's a journey to discover who this character will become. ... He doesn't come in at the beginning. He'll work his way up to become the head of the team."

Nobody else will know he has this weird medical condition, and it will take time for the team to find out.

Tassler, who said they haven't casted an actor yet for the role but are actively in discussions, wasn't caught by surprise to hear Petersen wanted out

"We've been talking about it since last year," she said. "You don't replace a guy like Billy, but add elements that invigorate and add alchemy to the show where it is today."

Petersen's character will depart after the upcoming season's 10th episode. He will remain an exec producer and stay committed to the series.

"Billy's an extraordinary guy," she continued. "Lets not forget his roots. He was originally in the theater and is now off to do a play. It's the artist's choice to change his life right now."