Jake Gyllenhaal believes everyone is a potential serial killer, including himself.The 'Donnie Darko' actor, who plays a police officer investigating the infamous San Francisco Bay Area murder case of the 60s and 70s in new movie 'Zodiac', is convinced most people could develop murderous tendencies. He said: "The idea of a serial killer is whatever we fear within ourselves. It's our own psyche, in a way."To me, the serial killer is obsession, and to me the serial killer is aspects of people's personalities that they don't want to deal with or that they indulge so much that it is dangerous."Gyllenhaal stars alongside Robert Downey Jr., who plays an alcoholic journalist reporting on the gruesome events.The Zodiac Killer murdered five known victims between December 1968 and October 1969.The murderer got his name from a series of taunting letters he sent to the local media until 1974, which included four cryptograms, three of which have never been solved.To this day, the identity of the killer is unknown, although the San Francisco Police Department recently reopened the case after marking it "inactive" in April 2004.