If “Out of the Darkness,” the CBS movie about the Son of Sam investigation, doesn’t ring true, it won’t be for lack of police expertise.

The film, to air Oct. 12, has ex-NYC detective Sonny Grosso as a co-producer, with technical advice from Ed Zigo, the zealous detective who pursued multiple murderer David Berkowitz to his 1977 arrest. Eddie Egan, a NYC detective-turned-actor (see “The French Connection”), plays the head of the Son of Sam task force.

Zigo, played by Martin Sheen, claims that Hollywood tends to glamorize “what is actually the rather tedious work of tracking down (criminals).” But “Out of the Darkness,” he said, avoids this by focusing on his personal life. (During the manhunt, his wife became ill and died.)

Zigo managed to keep “a lot of lurid details and swearing” out of the story, with the help of Sheen. When the writers got carried away, Zigo said, “Marty would stop (the filming) and say, ‘Zigo wouldn’t say that.’ ”