US actor Jake Gyllenhaal listens journalist's questions during a press conference on US director David Fincher's film 'Zodiac' at the 60th edition of the Cannes Film Festival.[AFP]

CANNES, France - Hollywood began its assault on the Cannes film festival's second day Thursday with "Zodiac", a movie about a notorious, true-life serial killer.

The movie reopens the unsolved mystery surrounding the eponymous murderer, never arrested, who terrorised California in the 1960s and 1970s while taunting police with letters and cryptogrammes sent to newspapers.

Starring Jake Gyllenhaal and Robert Downey Jr and directed by David Fincher, who made the thriller "Seven", the film is the first of the four US movies in the 22-strong field competing for Cannes's Palme d'Or.

The other features are "We Own the Night" by James Gray, "No Country for Old Men" by the Coen brothers and "Death Proof" by Quentin Tarantino, all to be screened later in the 12-day festival.

"Zodiac", which has already screened to solid reviews in the United States, eschews the usual serial killer format.

It sticks to the real-life facts and characters it is based upon so that, even though the presumed villain is identified to the audience, he is never arrested.

"I don't think it's a serial killer movie. I think this is a newspaper movie... a character study," said Fincher, after explaining that the blockbuster success of "Seven" had made him leery of becoming pigeon-holed in the thriller genre.

Although some of the murders are shown in unnerving detail, the focus of the film remains on the characters, especially Robert Graysmith (Gyllenhall), a San Francisco Chronicle editorial cartoonist who became obsessed with the Zodiac killer, and a crime reporter (Downey) at the paper.

Downey, whose electric style and past problems with drug abuse were brought to bear on his role, was a furiously inventive actor on set, Gyllenhaal said of his co-star, who was not present at Cannes.

"Some people would call that madness, I would call that genius," said the "Brokeback Mountain" star. Playing across from him was "kind of like jazz playing," he said.

Others in the cast said the script stood out for its intelligence and its determination to avoid Hollywood cliches -- and for the thinking touch of its director.

Chloe Sevigny, who plays Graysmith's wife, predicted the movie would go on to become "a great American classic".

Mark Ruffalo, who plays a cop in the picture, said he was impressed by the diligence of the Zodiac investigators who "followed the letter of the law, no matter what their guts said."

He added that the terror that Zodiac engendered during his reign and the professionalism of the police hunting him drew "an interesting comparison to make between this film and where we are today" in the United States.

"For me if you're going to get into terrorism and where we are today, you know maybe we should have been a little more diligent before going into war," he said, referring to the US administration's war in Iraq.

Fincher agreed the slayer's threats -- which at one point prompted police to follow San Francisco school buses around in 1969 to stop them being shot -- amounted to "a form of terrorism". But he said the film was not meant to be a commentary on contemporary events.