Thursday marks a big night on "CSI," but the real mystery is going on behind the scenes. Could America's top investigator be leaving TV's most popular crime unit? We've done our own investigation, and we've got answers.

You may have noticed there's been a lot less of William Petersen this season. At times the veteran actor has been an outspoken critic of CBS, and its two spin-offs, "CSI: NY" and "CSI: Miami," are fueling speculation that he could be leaving Las Vegas.

"It's a big television, you know, it has an amazing audience," Petersen said of "CSI."

And that audience averages 25 million viewers a week, making "CSI" the number one dramatic series on the tube -- a ratings powerhouse -- but is that power shifting? Petersen's co-star, Marg Helgenberger, recently told "Extra" about her promotion on the show, which puts her in charge of her own unit and on par with William's character. "I feel like I really scored, Gary and George on my team," Helgenberger had said.

But despite all the clues, "Extra" has uncovered the mystery of the missing Petersen. America's top crime investigator was been forced to cut back because of health issues in his real life.

But rest assured, "CSI's" chief is back at full strength starting Thursday night, and he's there to stay. In fact, in a report published just today, Petersen said, "I'm not going anywhere unless they want me to go."