FLASHBACK: A Look Back at 15 Years of 'CSI' Before Its Final Bow

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ET was with the stars of CSI: Crime Scene Investigation as they prepared for their emotional goodbye to the long-running CBS show, and we brought along footage from the early years for the cast members to reminisce about.

The groundbreaking crime procedural is ending with a two-hour series finale on Sunday, Sept. 27 after 15 years and more than 300 episodes.

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"It's very emotional," Ted Danson told ET. "Fifteen years -- that's what's great about this wrap-up episode."

ET's Cameron Mathison once guest starred as a suspect on the show and he was thrilled to go back to the set before its swan song, which will include original cast members William Petersen and Marg Helgenberger.

"I finished [the script] and went, 'Yeah, it's pretty good. I think the fans are going to like this,'" Helgenberger said. "Then I read the words 'End of Series' and I burst into tears on the plane."

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CSI debuted on CBS in 2000 and soon became the No. 1 show in primetime and made an impact overseas, becoming the most-watched show in the world for five years.

"I believed in the show from day one," Helgenberger said. "I thought it was going to be a hit, but to have the global impact that it's had … I wouldn't have anticipated that."

ET was with Helgenberger on set in 2001, where she made a similar observation.

"I always had a good feeling about this show from the time I first read it," she told us at the time.

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While the Emmy winner clearly stands by her statement, she denounced her haircut.

"That was an unfortunate hairstyle," Helgenberger joked.

Though CSI is ending for good, there is a silver lining for longtime fans of the series: Danson's character will live on, as he's moving on to CSI: Cyber.

"Wait a minute. Are they saying goodbye?" Danson joked. "Oh no! I've been so shallow all day long! I've been walking around with this big grin on my face! Oh no! Shoot."