'Castle' Canceled amid Tensions Between Stars Nathan Fillion and Stana Katic

The series finale is Monday

Photo: ABC

As Castle fans continued to process the news that star Stana Katic would be leaving the show after this season, ABC announced on Thursday that it is canceling the show after eight seasons.

PEOPLE confirmed that the Castle team previously filmed two different versions of the series finale, which will air Monday, that account for the show's, at the time, cancellation or renewal.

The cancellation news comes as rumors of tension between Katic and fellow lead Nathan Fillion have surfaced.

"The two were basically not speaking to each other if it wasn't in character," a source tells PEOPLE.

But they were speaking to their fans on social media after news that Monday's finale will be the series's last.

Fillion, 45, characteristically addressed the news with a few quips on Twitter: "Dear Fans, In whatever project I do next, loudly notice how unlike Castle and fresh and versatile I am. Thanks."

In another tweet, he wrote, "Dear Every Director, It appears I have an opening in my schedule."

Though Fillion's initial tweet proved more poignant: "All things must end. It has been an honor working with the men and women of Castle. I am truly amazed at what we've done. Thank you all."

Katic, 38, took a similarly gracious approach, posting a longer, handwritten note on Twitter.

"Your devotion to this show has carried us for these unforgettable eight seasons," she wrote. "I'm lucky to have met and worked with and for the lot of you. I will always be grateful."

Last month, Fillion wished Katic all the best after the news that she wouldn't continue on with the show.

"Castle has been one of the greatest joys of my creative life, and I hope the show continues on for years to come," he wrote on Instagram. "Stana has been my partner all this time, and I thank her for creating the character of Beckett who will live on for all of us as one of the greatest police officers on television. I wish her well, and have no doubt she will succeed in everything she pursues. She will be missed."

Katic also shared her thanks: "To the fans: thank you for your support. I love you," she tweeted in April.

Castle's series finale airs Monday at 10 p.m. ET on ABC.