LOS ANGELES — William Petersen is leaving “CSI: Crime Scene Investigation,” a move that might have fans of his brainy crimebuster Gil Grissom screaming bloody murder.

Petersen has agreed to return for occasional guest appearances and will remain a “CSI” executive producer, but his run as an original cast member will end this coming season, executive producers Carol Mendelsohn and Naren Shankar said Tuesday.

The air date for his final episode, the season’s 10th, has yet to be determined, but probably will be at the beginning of 2009, the producers said. The series debuted in fall 2000.

A number of familiar characters will return in the episodes building up to his departure; among them, former colleague and flame Sara Sidle, played by Jorja Fox; the Miniature Killer and sexy Lady Heather.

The decision to leave was Petersen’s, the producers said, declining to comment further. The actor had taken a break during the 2006-07 season to perform in a play.

The long-running show remains key for CBS: “CSI” finished last season as the network’s top-rated series, ranking No. 9 among all shows with an average weekly audience of 17 million. Spinoffs “CSI: Miami” and “CSI: New York” ranked 16th and 28th, respectively.

“CSI” is ready to deal with Petersen’s loss, its producers said.

“Shows lucky enough to go this long inevitably have to deal with this kind of change,” Shankar told The Associated Press.

A new male investigator will join “CSI,” a role that has yet to be cast.

The producers were mum about plot specifics but said Grissom will be reappraising his life after years of high-tech forensics investigations and after facing personal turmoil.

“We’re talking about a man who has suffered a great deal of loss recently… A man thinking about the next phase of his life,” Shankar said.

It’s wrong to characterize it as a mid-life crisis, which summons the image of someone who buys a Porsche as a solution, Shankar and Mendelsohn said; Grissom is confronting far deeper issues. “It’s Grissom in transition and Grissom asking, ‘Who am I?’” Mendelsohn said.

Shankar and Mendelsohn vowed that cast changes won’t change “CSI.”

“What makes shows go off the rails is they forget who they are. We’re a crime-mystery-forensics drama” with a focus on exceptional cases, Shankar said.

“Whoever comes in and joins the team after Grissom is going to be a different guy. But the nature of the show and what fans get out of it, that’s not going to change,” he said.