‘CSI’ Series Finale SPOILERS: Original Cast Starts Filming ‘Immortality’ And Lady Heather Gets Ready For Her Close Up [PHOTOS]

"CSI" will solve their last case when CBS airs a two-hour series finale in September and filming has begun on the episode titled, "Immortality." The cast and crew are documenting their final days on set with group photos and other fun behind-the-scenes looks.

Marg Helgenberger and William Petersen, who will reprise their roles as Catherine Willows and Gil Grissom, posed for a lovely photo as "Grillows" and the entire original cast (minus George Eads and Gary Dourdan, who are not returning) squeezed in for another pic.

#Grillows with Jorja & Paul backing us up. #CSI4EVER! pic.twitter.com/jWdZOzxYC2

— Marg Helgenberger (@MargHelgen) July 31, 2015

#OGCSI Great fun today with this gang! pic.twitter.com/HHAx9norkj — Marg Helgenberger (@MargHelgen) July 31, 2015

Creator Anthony Zuiker also has shared photos during filming and revealed a few clues as to what fans can expect from their favorite crime solvers. Plus, he posted a first look at Melinda Clarke who returns as the popular recurring character, Lady Heather Kessler.

Also see Catherine's grown up daugther Lindsey, played by "Faking It" star Katie Stevens, and Wallace Langham a.k.a. Hodges making trouble for "CSI's" OTP, Grissom and Sara, once again.

Check out all the photos from the "CSI" set in the slideshow above.

"CSI" will premiere its two-hour finale on Sunday, Sept. 27 at 9 p.m. on CBS.