Although Fox quit the show this year, Mendelsohn told EW.com the actress will be back playing Sara Sidle in "multiple" episodes starting with the season opener in September.

Also to appear in that episode is outgoing "CSI" cast member Gary Dourdan, who plays Warrick Brown.

"We're not saying whether or not Warrick lives or dies," Mendelsohn told the entertainment news Web site. "But we will say that the premiere may be the last time our loyal fans see the entire original 'CSI' cast together."

As for Petersen's upcoming departure in the middle of next season, Mendelsohn told EW.com the move did not come as a surprise to the "CSI" creative team.

"We do have a plan," she promised, saying an A-list actor will join the cast, playing a professor with a background in pathology who becomes a crime scene investigator.

EW.com said Kurt Russell, John Malkovich and Laurence Fishburne were said to be under consideration.

Mendelsohn, however, would only confirm Malkovich was approached about the role, but had scheduling conflicts.

Petersen is expected for occasional guest appearances, EW.com said.