Joe Mantegna, who joined the CBS procedural as David Rossi in the third season, wrote a touching letter to the fans on his final day of filming on the series, calling his 12-year run an unbelievably rewarding time in his career and life.
"I can't help but reflect in awe at what a truly wonderful experience it has been," Mantegna, 71, wrote in the letter posted Monday. "I was welcomed by the cast and crew instantly, making me feel part of the team on [Day 1]. The next 12 years would be a wild and wonderful ride. Yes, I will miss playing David Rossi, and I'll miss the wonderful writing that brought him to life, but mostly I will miss seeing my fellow castmates and our fantastic crew everyday."
"In 15 seasons we've all grown a little older, hopefully a little wiser too. We laughed together, we cried together, and like a family we gained and lost members along the way," he reminisced. "Some of our family went on to other shows and jobs, and sadly, some we lost too soon to the heavens." Key cast departures over the years have included Mandy Patinkin, Thomas Gibson,Shemar Moore, Rachel Nichols, Jeanne Tripplehorn, Jennifer Love Hewitt and Damon Gupton.
In reflecting on his Criminal Minds run, Mantegna said he is "satisfied with what we accomplished and proud of the work," adding that he is "excited for what's next." He closed out his emotional letter by thanking the fans who have stuck by the show through the years "for being the most loyal, tolerant, and passionate fans we could have ever asked for."
Mantegna's co-stars also took to social media to chronicle their goodbyes as they began to wrap.
Aisha Tyler marked her five-season run as forensic psychologist Tara Lewis in the most dramatic fashion -- by getting a gun tattoo to commemorate her time.
"This weekend the entire cast and crew danced and toasted and laughed and celebrated 15 seasons of a powerhouse show made incredible every day via the hard work of an incredible creative family," Tyler, 48, shared on Instagram, calling the end of Criminal Minds "a magnificent end to a truly magnificent thing."
"Life is now. Happening all around you. In the little joys and the big triumphs. In the bittersweet farewells and the exciting new friendships. In life-changing experiences you've had and new adventures that await," she wrote.
Following the season 14 finale in February, which featured a bombshell confession by JJ (Cook), who confessed to Reid (Matthew Gray Gubler) that she has "always" loved him, showrunner Erica Messerrevealed to ET that that revelation "will definitely be explored" in the final run.
"Her confessing that doesn’t mean she doesn’t love the life she has tremendously. Without giving away too much of the actual dialogue that’s coming up, she’s been defined by the love in her life and the love in her life is Will and the kids, it’s Reid, it’s her friends, it’s the work that she does," she previewed. "There are a lot of things that define people and [Reid] is one of those things to her. Maybe if she had never met Will in season two, things would’ve been different. If in another time and place... There’s heartbreak to that but you would want to know that in a friendship or relationship. It’s complicated."
For more on the final season of Criminal Minds, see ET's complete coverage below.