Two films based on the Fox drama were released in 1998 and 2008, but plans for a third cinematic version were put on hold when The X Files: I Want to Believe struggled at the box office.
"The second movie did not perform the way anybody wanted it to," Spotnitz admitted to Collider. "I'm proud of that movie, but it makes sense to me that it didn't [do well].
"It was released at the height of summer, and it was a story-of-the-week. That's not what the movie-going audience wanted. The movie-going audience wanted the aliens. That's what they know The X-Files for."
Spotnitz - who worked on the original show between 1996 and 2002 - argued that the central story arc of The X-Files is "not done, and... should be finished".
"I don't think it's too late, but I think it's gonna be, pretty soon," he claimed. "I'm still agitating with everyone I can grab to say, 'Let's make this movie while we still can!'
"I think everybody should write to 20th Century Fox. I've been saying for years now that I feel it's a cultural crime that they have not finished the series."
> X-Files producer on third film: It'd be a shame if it didn't happen
> Lance Henriksen expects Millennium film
Watch the trailer for The X Files: I Want to Believe below: