W arner Bros. and director Peter Jackson are seeking to turn the planned two-film "Hobbit" franchise into a trilogy, a move that could pay off in bigger box-office returns, according to people familiar with the situation.

The Burbank film studio originally planned to release two "Hobbit" movies based on J.R.R. Tolkien's literary prelude to "The Lord of the Rings" trilogy, and principal photography on those two pictures finished in New Zealand this month. The first is set to come out in December, followed by the second in December 2013. The two films combined cost about $500 million.

But Jackson has concluded that there is enough material from the book, as well as the extensive appendixes to "The Lord of the Rings," to make a third film, according to three people who were not authorized to speak publicly. New Line Cinema, the Warner Bros. unit overseeing production of the movies, is eager to see it happen, and talks are under way with actors and others who would need to sign off on the plan.

A third film is far from a certainty, however, because there are numerous rights-holders and actors with whom new deals must be made. Lead actors in particular hold leverage as they know New Line would need them for the picture. (Actors and some rights-holders had previously made commitments only to two pictures.) Talks with a number of the franchise's actors -- including Martin Freeman and Ian McKellen -- have been taking place in the last few weeks.

New Line President Toby Emmerich did not return a call for comment, and Jackson could not be reached for comment.

With its blockbuster "Harry Potter" and Christopher Nolan-directed Batman franchises both now concluded, Warner Bros. is hungry for franchise pictures. It has a handful of tent-pole movies set for release next year, including the Superman movie "Man of Steel" and Guillermo del Toro's sci-fi picture "Pacific Rim." But so far it has only one film of note on its 2014 calendar -- a movie based on the Lego children's toys.

What plot details would be held back for the third "Hobbit," and how the first and second would be shaped as a result, is a critical question for fans. Of particular interest is whether Jackson would hold back the climactic battle that takes place at the end of the "Hobbit" book for the third movie.

If Jackson were to keep that battle in the second film, the third movie could center on material from the "Lord of the Rings" appendixes, which have not previously been adapted for screen. (The six appendixes ran with "The Return of the King," the final book in the trilogy, and feature backstories on many of the characters and cultures in Middle Earth.)

It's not clear how much additional shooting Jackson would have to undertake, or whether the additional picture could largely be crafted through artful editing of existing footage, which would bring the cost down. Principal photography has been finished on the heretofore planned two films, with four weeks of reshoots and additional footage being filmed now.

Serious conversations between Jackson and New Line for a new film began about two weeks ago, around the time the director visited California for a promotional appearance at Comic-Con International in San Diego. The director acknowledged in interviews at the fan festival that he was considering the possibility of extending "The Hobbit" in some form. In an interview with the fan blog Hit Fix, he said, "We have certainly been talking to the studio about some of the material we can't film, and we've been asking them so we can do a bit more filming next year," adding, "I don't know what would come of that, whether it'd be extended editions or whatnot."

The franchise is seen as a comparatively safe bet in the risk-laden movie business. The movies in Jackson's "Lord of the Rings" trilogy, a fantasy franchise that focused on many of the same characters, grossed nearly $3 billion worldwide when they were released in 2001, 2002 and 2003.

But Jackson is taking some gambles with "The Hobbit," particularly a decision to shoot at 48 frames per second, rather than the standard 24. That technical switch is intended to give the picture a smoother, more realistic look. Footage shot in the new frequency elicited a mixed reaction at the industry trade show Cinema Con in April.

Due to the complicated rights history of "The Hobbit," a number of stakeholders must agree to the plan for a third picture.

Independent studio Metro-Goldwyn-Mayer controls 50 percent of the film rights and international distribution, though it has contracted with Warner Bros. to handle those duties on the first two movies.

Producer Saul Zaentz has a long-running participation claim, as do Harvey and Bob Weinstein. The independent film moguls originally developed the "Lord of the Rings" movies when they ran Miramax Films.

The Tolkien estate receives a share of the earnings from the movies but does not need to grant permission for additional productions.

Breaking a single book into multiple movies has come into vogue in recent years as studios seek to wring value out of popular material.

The "Harry Potter," "Twilight" and "Hunger Games" franchises all have split single books into two pictures. But there is no known precedent for turning one book into three films.

What's more, in all those instances, the decision to create an additional movie out of an existing book was made before production began, not after it had wrapped.