The death in May of Paul Reyes, one of the leaders of the Colombian terrorist group, Farc (the Revolutionary Armed Forces of Colombia), lends a timely appeal to Victor Gaviria's new project about the origins of the movement.

The Colombian director, probably the best known film-maker to emerge from the territory, is working on Black Blood - The Hour Of The Traitors (Sangre Negra - La Hora De Los Traidores), which may turn out to be his most controversial film to date.

It is based on a true story about a young Farc leader (not Reyes) betrayed by the closest members his own family in exchange for a National Government reward.

"Farc is the major reality of my country, other than drugs," says Gaviria. "I want to investigate the process that turns these people into bandits who are going around creating almost a genocide."

The film-maker made his name in 1990 with Rodrigo D: No Future, a brutal film about a teenager drawn into gang warfare in the Colombian drug capital, Medellin. It was the first Colombian film to play in Cannes (in Competition in 1990), and his 1998 feature The Rose Seller, about a 13-year-old girl trying to survive on the streets, also went on to screen in Competition.

"The two times I've competed in Cannes are what make me stand out as a Latin-American director," says Gaviria.

He suggests his new film will be "like a western", but will reflect the reality of what is going on in Colombia. It will show how the bandits became politicised and threw in their lot with the communists, and how the Farc was first set up in the mid-1960s.

The $1.5m project has been set up as a co-production between Colombia and Spain. Madrid-based Kevin Williams Associates is handling world sales on the project, which is being produced by Fehrmann Productions in conjunction with Carlos Guerrero, Sixaola and World Wide Media Investments. Financing is complete and the project is due to begin shooting in the early autumn. "We're going to try to work in places that are most similar to where the Farc are located. But obviously we don't want to be too close to the Farc!" the director says.

All territories are available, including Colombia (although there is a TV deal there). "The deal with Colombia has been left open so we can do an all-rights deal with a studio if necessary," says Kevin Williams.

As is usual on his films, Gaviria will be working with non-professional actors. The difference this time is that some of the cast members will be ex-guerrillas who have laid down their weapons. "There are hundreds of them who've been rehabilitated," says Gaviria. The director will spend several months training his new charges. "The process is slow. That's why there is so much pre-production."

The director acknowledges the plight of the Colombian politician (and French citizen) Ingrid Betancourt, kidnapped by the Farc six years ago and still held hostage, is likely to heighten international interest in his film. "Everybody wants to know what the situation was that caused her to be kidnapped and what the social situation in the country is that allowed this to happen. It's good for the movie, but it's obviously bad for her."