NEW YORK (AP) ― For much of the nine years that Shane Salerno worked on his J.D. Salinger documentary and book, the project was a mystery worthy of the author himself.

Code names. Hidden identities. Surveillance cameras. Until 2010, when “The Catcher In the Rye” novelist died at age 91, only a handful of people were fully aware of what he was up to. Even now, with the release date of the film “Salinger” less than three weeks away, little is known about a production that draws upon more than 100 interviews and a trove of documents and rare photographs, and that promises many revelations about an author who still fascinates millions.

“I have worked more than 200 documentaries in my career and Salinger was the most secretive and the most intense film I have ever worked on,” said Buddy Squires, the film’s cinematographer and co-producer who has worked on such Ken Burns documentaries as “Jazz” and “The Central Park Five.”

“This film was not run like a film production,” said Jeffrey Doe, a co-editor and co-producer. “It was run like a CIA operation. Everything was compartmentalized, top secret and on a need-to-know basis. It was really intense.”

More than three years after Salinger’s death at his New Hampshire home, numerous questions remain unanswered, notably what ― or if ― he wrote during the self-imposed retirement of his final decades. The new Salinger book and movie are not the first projects ever billed as cracking the Salinger code, and the author’s literary estate did not participate. But Salerno has won some important converts.

The Weinstein Company quickly signed up the movie after seeing it earlier this year, as did PBS, which reportedly paid seven figures and will air the documentary in January as the 200th installment of its “American Masters” series. Simon & Schuster reportedly paid seven figures for the book, which runs 700 pages and was co-authored by Salerno and David Shields.

The film, which opens Sept. 6, is expected to be shown on more than 200 screens nationwide, a high number for a documentary. The book’s planned first printing is for more than 100,000 copies.

Salerno, 40, is best known as a screenwriter, with credits that include “Savages” and a planned sequel for “Avatar.” Salerno, who declined to be interviewed, reportedly spent some $2 million of his own money for the project and traveled around the country and in Europe to research it.

As if internalizing the Salinger legend, he made secrecy not just a priority, but an obsession. Virtually everyone involved had to sign non-disclosure agreements, including Shields, Doe and Squires, and even Squires’ wife. At Technicolor, where post production took place, the film was called “Project Y” and stored in a vault, as if in homage to the vault where Salinger allegedly stored unpublished manuscripts. The Technicolor vault was kept in a room under the watch of seven surveillance cameras.

The film’s ending was added just in the past few days.

“Everything was on a very strict ‘need to know’ basis and the only person who knew everything by design was Shane,” Squires said. “Crew members knew about their part and not other parts of the film. Some crew members brought on for a particular sequence walked away thinking they made a film about World War II or 1940s Hollywood or Charlie Chaplin.

“This was the first film in my career,” he added, “where I checked into hotels under a fake name.”

There have been reasons all along to value secrecy. The Salinger crew worried that early publicity would make some interview subjects reluctant to talk. They also cited the example of the Michael Moore documentary “Sicko,” which leaked online in advance of its release. For “Salinger,” emails were often sent under fake headers and online correspondence in general was minimized. Whether working on the book with Shields, or recruiting associates for the film, Salerno preferred handling business in person.

“When Shane first called to hire me, he did not tell me the subject matter of the film. He just said ‘If I get you a plane ticket, will you come to Los Angeles to talk with me about a film project?’” Squires said. “After I arrived in Los Angeles and signed a phone book size confidentiality agreement, Shane revealed to me that he was underway with a film about J.D. Salinger.”

“For years I couldn’t tell anyone what I was working on,” Doe said. “It wasn’t just friends. My own family didn’t know!”

Access was restricted even for the movie’s soundtrack composer, Lorne Balfe, who won a Grammy for his score for “The Dark Knight.” In liner notes he wrote for the “Salinger” soundtrack CD, Balfe recalled that his job was “very complicated” in part because he had to score many scenes he was not permitted to see.

Once press screenings begin for “Salinger,” and copies of the books are shipped to stores, keeping all the secrets could prove highly challenging, especially in the age of Twitter. Weinstein publicity head Dani Weinstein (no relation to company founders Bob and Harvey Weinstein) said journalists seeing the movie will be asked to sign an embargo agreement. Jonathan Karp, who heads Simon & Schuster’s flagship imprint, said the publisher was taking “extraordinary measures” to ensure early copies were not sold or obtained.