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."

It sounds like something that will either be very good or very ... not so good. But I give 'em all credit for derring-do and willingness to try something totally off the wall -- a scribe swap that cuts across genres, and even studios.

"CSI" writers Sarah Goldfinger and Evan Dunsky have penned the May 5 seg of "Two and a Half Men," based on a story by "CSI" czarina Carol Mendelsohn and Naren Shankar. Chuck Lorre and Lee Aronsohn, co-creators and exec producers of "Men," have done the script for the May 8 installment of "CSI."

The "Men" seg, "Fish in a Drawer," involves a CSI team coming to Charlie Harper's beach house to investigate -- what else? -- a mysterious death. It should be further enlivened by guest stars Robert Wagner and Jenny McCarthy.

"CSI" seg "Two and a Half Deaths" revolves around the death of a high-maintenance sitcom star, played by Katey Sagal, while shooting her show on location in Sin City. Suspects include the entire writing staff, her loser husband and her stand-in, played by Rachael Harris. 

I'm thinking positive and hoping these experiments be as playful as they outta be. I will do some more investigating and find out whose DNA is all over this idea, what the motivation was and the weapon (laptop, legal pad, etc.).

In the meantime, here's some more pics of the crossover episodes.

(In the pic above, that's Aronsohn leaning over the body, and Lorre directly behind him.)

From the "CSI" - "Two and a Half Deaths" seg: