It’s a question that’s fascinated audiences since “Casablanca’’ premiered in New York City 70 years ago this month: What ever happened to Rick and Ilsa? Was saloonkeeper Richard Blaine, played so iconically by Humphrey Bogart, ever reunited with his beautiful ex-lover Ilsa Lund (Ingrid Bergman), whom he nobly sent off on that plane to Lisbon with her husband, resistance fighter Victor Laszlo (Paul Henreid)?

Now a producer whose grandfather was a Warner Bros. co-founder, and whose uncle set the original film in motion, is trying to get the studio to finally make a sequel — based on a treatment written more than 30 years ago by Howard Koch, one of three writers who shared an Oscar for the original script.

There have been several attempts over eight decades to continue one of the most beloved films of all time. Listed at No. 2 among Hollywood’s greatest movies in a 1998 survey by the American Film Institute, “Casablanca’’ has been in heavy rotation on TV, in repertory houses and on video for decades.

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It’s rivaled only by “The Godfather’’ as the most quotable: “I’m shocked, shocked that gambling is going on in here!’’; “Of all the gin joints, in all the towns, in all the world, she walks into mine’’; “Here’s looking at you, kid’’; “I’m no good at being noble, but it doesn’t take much to see that the problems of three little people don’t amount to a hill of beans in this crazy world,’’ and, of course, the one the makers of “Casablanca 2” are banking on: “You must remember this . . .”

“Casablanca” had its gala world premiere (with a military parade) on Thanksgiving Day 1942 at the old Warner Hollywood Theatre on West 52nd Street (now the Times Square Church).

It was a big hit when it opened nationally, and it won the Oscar for Best Picture of 1943.

Not long afterward, Warner Bros. announced a sequel called “Brazzaville’’ — after the location of the Free French garrison mentioned in the last scene.

But this “beginning of a beautiful friendship” only got as far as a treatment by Frederic Stephani.

Picking up after Rick heads off into the fog with Capt. Renault (Claude Rains), it’s revealed after the American invasion of Casablanca that Rick and Renault were secretly Allied agents all along.

“The moment Rick becomes . . . an agent of the secret police, the interest in his position and character largely evaporates,’’ wrote Frederick Faust, a Warner contract writer asked to evaluate Stephani’s proposal.

“Brazzaville” was scrapped.

The nearest Warner Bros. ever got to a big-screen sequel was “Passage to Marseilles,’’ a 1944 follow-up starring Bogart as a Free French journalist who escapes Devil’s Island to battle the Nazis. The cast included “Casablanca’’ veterans Rains, Sydney Greenstreet and Peter Lorre.

Rick finally resurfaced — on the small screen — in 1955, played by actor Charles McGraw. One of three revolving segments of “Warner Bros. Presents’’ series based on old movies, “Casablanca” appeared every third week, with Richard Blaine still operating his Cafe Americain in Morocco 15 years later.

This was a less cynical Rick, a Middle Eastern Mr. Fixit who protected an Arab orphan and helped an elderly Englishman find who betrayed his son killed in the war.

“Casablanca” did not fare well with either audiences or critics and was canceled after seven months.

Re-enter Howard Koch, who had been blacklisted in the late 1940s at the behest of Jack L. Warner because of his work on the pro-Soviet film “Mission to Moscow” — which Jack and his older brother, Harry, the company’s president, had begged him to write in the first place.

“My father was very philosophical about that,” Koch’s son Peter recalls. “He always said, ‘If I hadn’t been blacklisted, then I wouldn’t have gotten to live in England.’ ”

Warner Bros. was out of the founding brothers’ hands for more than a decade when Howard Koch began working on a treatment for a sequel around 1980.

I was interviewing Koch at his home in Woodstock in 1982 when he mentioned his annoyance at plans for a second “Casablanca” TV series starring David Soul of “Starsky and Hutch” fame as Rick and Scatman Crothers as Sam.

“I think it’s a mistake, because ‘Casablanca’ without Bogie, Bergman and Claude Rains just won’t be ‘Casablanca,’ ” Koch told me.

He was doubly irked because his former employers had turned down his idea for a sequel just a year earlier.

Koch said he proposed a story revolving around the son of Rick Blaine and Ilsa Lund — “conceived the night he renewed his affair with Ilsa in Casablanca” — and the young Richard’s efforts to find his father, who disappeared during the war.

“I plunged the son into the Middle East today, and I proposed getting the actors from the cast who were still alive — including Ingrid Bergman — to play older versions of their characters,” Koch told me. “I felt it would have been an interesting thing to do without violating ‘Casablanca.’ ”

The second TV series, which took place before the events of the movie, premiered in April 1983 on ABC and was canceled after three episodes.

Koch, meanwhile, continued working on his own sequel, writing out Ingrid Bergman after she died of cancer in 1982.

In a 1988 synopsis for what he then called “Return to Casablanca,” Koch writes that Ilsa and Laszlo “attempted to locate Rick after he and Renault left to join the Free French forces opposing Rommel in North Africa. They have had no success.

“After leaving Casablanca for America, Ilsa learned she was pregnant,” Koch continued. “She gave birth to a boy who grew up in America. The real father of the boy, it turns out, was not Laszlo but Rick.

“He was conceived the night Ilsa came to Rick’s place to plead for the Letters of Transit . . . The secret was not kept from Laszlo, but being the kind of man he was and owing so much to Rick, he adopted the child and treated him as his own son.

“The boy was named Richard, and he grew up to be a handsome, tough-tender young man reminiscent of his father. He had been told the truth about his origin and has a deep desire to find his real father, or at least more about him, since Rick’s heroic at actions in Casablanca have become legendary.’’

The main action of the film takes places around 1961, as the 20-year-old Richard arrives in Casablanca after the death of his mother and Laszlo.

“Richard finds himself very much a stranger in the Arab world, a world now under Arab rule since the expulsion of the Germans and Vichy French who occupied Casablanca during the war,” Koch wrote.

An elderly waiter who once worked for Rick takes the son to a rubble-strewn lot where they find a partly burned wooden sign.

“He strikes a match, holds up the sign for Richard, who can barely make out the word ‘Americain’ — the rest is illegible,’’ Koch wrote. “At this point, we flash back to the final love scene between Rick and Ilsa in the original ‘Casablanca.’ ”

The waiter explains that the Nazis blew up the cafe after Rick, Ilsa and Laszlo’s escape. And that now, in 1961, a “citizens movement” led by an Arab woman who calls herself Joan is leading “guerrilla warfare’’ to track down “Nazi-led outlaws.’’ Richard eventually discovers his father’s fate.

Peter Koch says young Richard’s love interest was likely inspired by the activist singer Joan Baez, “who my dad greatly admired.”

“I’m not sure they ever met, but he knew Bob Dylan, who lived in Woodstock at the time, quite well,” Peter adds.

Warner Bros. turned the second treatment down in 1989.

But they weren’t done with “Casablanca.’’ Nine years later, the studio’s corporate sibling Warner Books published “As Time Goes By,’’ a novel by former Time magazine music critic Michael Walsh.

This was a both a prequel — imagining Rick as a Jewish gangster in 1930’s New York (original name Yitizik Baline) — and a sequel following directly after the movie.

Rick and Renault go to London, where they end up getting recruited by Laszlo and Ilsa for a top-secret mission, the assassination of Nazi leader Reinhard Hydrich in Prague, the most significant act of Czech resistance during World War II.

Howard Koch died in 1995, but back in 1988, he had taught a screenwriting class in Santa Barbara. Among his students was Cass Warner, granddaughter of Harry Warner and grand-niece of Jack L. Warner.

“We instantly became like family when he discovered who I was and that I was fascinated by his tories of working on the lot as I was writing a book on the family then,” she recalls. “He loved working at Warner Bros. despite my great uncle Jack throwing his name onto the McCarthy blacklist.”

Visiting his home in Woodstock, “I discovered stacks of unproduced screenplays and treatments,’’ Cass Warner says.

“He said he couldn’t get representation as he was told he was too old,’’ she recalls. “Without hesitation, I volunteered to try my best to get his works produced.’’

Cass Warner directed a documentary based on her book, “The Brothers Warner,” that was released in 2007. Her production company, Warner Sisters, had hoped to set up “Return to Casablanca” for the studio’s 90th anniversary next year.

“Warner Bros. passed on it a year, a year and a half ago,” she says. “But they indicated they were willing to revisit this if I could find a filmmaker they were interested in working with.”

Warner is still hopeful.

Aljean Harmetz, who wrote the definitive history of the film, “Round Up the Usual Suspects,’’ says Koch’s treatment sidesteps “the big problem with all of the previous sequel attempts — the impossibility of finding actors who can play characters so closely identified with Humphrey Bogart and Ingrid Bergman.”

Harmetz says even casting the son would be a challenge. “There really are not many solid 20-year-old actors,” she says. “Certainly not Robert Pattinson, though maybe that guy who’s popping up everywhere, Joseph Gordon-Levitt.”

Film historian Leonard Maltin is also cool on the sequel idea — but is impressed that “Return to Casablanca’’ comes from one of the film’s original creators, Howard Koch.

“I don’t want anybody messing around with my favorite movie in any way, shape or form,’’ Maltin says. “But if it were to happen, I supposed I could bear this extension of the story, especially since it came from the right source.”

But don’t even suggest the idea of a sequel to Stephen Bogart, the 64-year-old son of Humphrey Bogart and Lauren Bacall.

“If they want to make a movie about intrigue in North Africa, that’s fine, but bringing the specter of one of the greatest movies of all time into it makes it just another feeble attempt at ‘Casablanca 2,’ ” he says.

“There are certain films, like ‘The Wizard of Oz,’ ‘Gone With the Wind’ and, of course, ‘Casablanca’ that need to stay as pristine and perfect as they are.’’

Or, as his father’s character so memorably put it, “We’ll always have Paris.”