It is a bright, chilly day in November and Nelson DeMille is at the beach. He stands on Fire Island near the sleek black granite memorial to the 230 victims of TWA Flight 800 and points toward the horizon, where the steel-blue Atlantic meets a cloud-puffed, powder-blue sky.

"That's about where the plane went down," says the author of "The Gold Coast" and "The General's Daughter," now an expert on the 1996 crash eight miles off the coast of Long Island.

Despite a lengthy government investigation, the downing of Flight 800 is still a subject of controversy. Did the plane blow up because of mechanical failure, as the official investigation concluded, or because of friendly fire from Navy maneuvers? Or was the crash caused by a missile launched from below by terrorists, as suggested by a streak of light shooting upward that was seen by more than 200 real-life eyewitnesses - and recorded on videotape (fictionally) by two illicit lovers in DeMille's new novel, "Night Fall"?

An expected bestseller

The novel, which debuts this week with a million-copy first printing, is based on the true story, presented - in terms of basic facts - accurately. But DeMille's mystery mostly focuses on his fictional hero's search for a wholly imagined adulterous couple and their videotape.

The literary approach came to him long after Flight 800 first captured his imagination. The event had touched his life from the moment it happened.

"This plane crashed on July 17, which was a Wednesday, and, on the Sunday before, I put my daughter on the same flight, TWA 800 to Paris, with a bunch of students going to Paris to study," recalls DeMille, 61, who grew up in Elmont and lives in Garden City. As on July 17, his daughter Lauren's flight was late leaving, putting it over Long Island at a time when an El Al flight was scheduled to be there.

DeMille also may have heard the explosion: "I was out on the North Fork for the whole summer, writing 'Plum Island.' When the plane crashed, I was on the back deck, and I think I heard something, though I'm not quite sure." He thought it could be fireworks, as did others who heard a boom. "I was about 20 miles away, but it's not unusual for sound to travel 20 miles - a sound like that," he adds.

"Then somebody called me and said, 'Did you hear about TWA Flight 800?,' and it kind of made my heart stop," because that was his daughter's flight. "I said, 'No, why?' and the woman said, 'It just crashed.' I turned on Channel 12 news and got early reports of a mid-air collision. A lot of the early reports had to do with eyewitnesses seeing what they described as a rocket or a missile rising from the water right before the plane exploded. So it seemed everybody at first was saying it was a terrorist attack."

Terrorist scenarios

He thought of writing the book a few years later, when he was researching "The Lion's Game," his 2000 thriller about Middle East terrorism (which presciently discusses the possibility of terrorists using planes to attack the World Trade Center). Several members of the federal Joint Terrorism Task Force he interviewed told him, off the record, that they had helped with the Flight 800 investigation and weren't satisfied with the final report's conclusion of malfunction.

And neighbors of DeMille's in Garden City came to him, after reading a media report that he was thinking of writing a novel based on the incident, to say that they had been near the site of the explosion in a boat and had seen a flash of light before the crash.

Having people come forward is an "advantage of being a bestselling novelist," says DeMille, who mentioned his newest project in interviews. "I said what I was working on, and a lot of TWA people and retired FBI people contacted me. I used to have to go look for research sources, but now they come and find me."

He did a lot of other research, too, reading government documents, interviewing eyewitnesses and talking to FBI and police investigators on the task force.

Writing this kind of "what if" book, says DeMille, is risky because events, such as another terrorist attack or the emergence of new evidence on Flight 800, can overtake it. Philip Roth's current bestseller, "The Plot Against America," which imagines Charles A. Lindbergh becoming president in 1940, is "alternate history," DeMille says, "but it's in the same vein, in that you're taking reality and going off in a different direction with it."

His book's hero is John Corey, a former NYPD cop who is now a member of the Anti-Terrorist Task Force (DeMille's variation on the name, though he keeps it at its real address, 26 Federal Plaza, 26th floor). The wise- cracking, street-smart, anti-authoritarian Corey was also the hero of "Plum Island" and "The Lion's Game." (He'll return in yet another novel, the one after next, says DeMille, in a story that wraps up events in "The Lion's Game.")

DeMille credits his son, Alex, who works in film, with coming up with an idea to end "Night Fall" on Sept. 11, 2001, two months after the fifth anniversary of the crash, when the fictional Corey first becomes interested in its cause.

Corey gets involved in his own renegade investigation after his wife, an FBI agent, takes him to a memorial service for the victims' families. DeMille missed the real-life ceremony, so he had to rely on descriptions in Newsday and the New York Times, he says.

But he visited a Coast Guard station, the hangar where the remains of the downed plane were reconstructed and the Hamptons beaches and communities where he sets his novel. DeMille also stopped by the memorial at Smith Point County Park, with its landscaping of natural grasses and trellises of silver cedar, and was visiting it again with a reporter.

There, on a curved black granite wall filled with the victims' names, DeMille notes that of the pilot, Ralph G. Kevorkian. Kevorkian, he says, was a friend of one of DeMille's best friends, also a TWA pilot, who had flown the same aircraft from Paris two days earlier. On a lower wall that tells the story of the flight and of the monument, DeMille notes that the text calls what happened to TWA Flight 800 "an explosion of controversial origin."

Carefully chosen

John Seaman, the man who wrote the text, is the chairman of the board of The Families of TWA Flight 800, the nonprofit group that raised the money to build and maintain the memorial. He's at the park this day overseeing expansion plans and seawall work to prevent erosion from damaging the memorial.

His phrase carved in stone, Seaman says, was carefully chosen, reflecting "something all the families took comfort from. ... I just left it at that, not a bomb or this or that." Seaman, who lost his 19-year-old niece and godchild, Michele Becker, in the crash, says the catastrophe produced a stream of books and TV shows. New information continues to emerge, such as recent reports, uncovered through the Freedom of Information Act, that various terrorist groups have taken credit for the explosion.

"It's still a mystery," says Seaman, 55. Could DeMille's book, which he hadn't seen, be controversial? "Other things written or filmed about Flight 800 have been viewed differently by different family members," he says. "Some people have suggested no one should talk about it, that it's hurtful. But most recognize that it's a mystery that will never be solved to everyone's satisfaction."

He continues: "If there was strong evidence to believe it was a deliberate act, we would all be consumed with trying to find out who is responsible, and that could take years and years. ... We would feel that person should be caught and brought to justice."

Several citizen groups have continued efforts to shed more light on the crash, including Flight 800 Independent Researchers Organization, formed in 1999 and headed by Tom Stalcup, a physicist who owns a small electronics company in Falmouth, Mass. With 40 members, including scientists and former crash investigators, the organization gets a respectful mention in DeMille's book.

One who doubts

Stalcup, 34, says he was a graduate student in Florida when he watched television reports and became skeptical about the government's investigation, especially an animated version of the crash (also referred to in the novel) that explains the upward flash of light reported by many witnesses as an optical illusion that occurred after the explosion rather than before it.

His group is suing in federal court for evidence of foreign objects in victims' bodies that it believes has been withheld. He's received e-mails, he says, from about a dozen people who believe the flash of light was captured on a videotape shown one time, on one channel, right after the crash, before disappearing. "I don't know if someone is trying to get us off the track," he says of that story.

The National Transportation Safety Board, which issued its final report in 2000, isn't planning to back down, says spokeswoman Lauren Peduzzi. "We're always open to new scientific information, but at this time we haven't received any. I don't know how much help we can be on a fictional report," she says, referring to DeMille's novel. There have been at least three other fuel tank explosions since Flight 800 went down, she adds. (The most recent ripped apart a Thai Airways Boeing 737 in March 2001.)

"We won't speak to the theories," says Peduzzi. "We did an exhaustive scientific investigation" that started in conjunction with the FBI's task force. "Once they found no evidence of criminal activity, they backed off."

But the questioners of the FBI haven't, and at least one welcomes DeMille's novel. Mark L. Berry, a member of Stalcup's group and an airline pilot who once worked for TWA and who lost his fiance on Flight 800, writes via e-mail that he thinks DeMille "is the perfect author (based on the style of his books that I have read) to bring renewed interest in TWA 800 through an exciting novel that hints at a deeper truth buried in his fictitious tale."

Will families be troubled?

Stalcup says he thinks some family members may be offended by DeMille's book, which he hadn't yet seen, "and some of the officials in the government may be upset that this is getting attention again. I think they just want it to go away."

CIA operatives may be mollified by DeMille's next book, which he says will reverse the contempt "Night Fall" shows for them. It will be about a suave CIA agent, an American James Bond-type, he says, who will pursue a terrorist plot to use an atomic bomb on an American city.

DeMille says he thinks family members will look on his Flight 800 book favorably, though he understands many "would like to believe it was an accident." He also doesn't want to be seen as a "conspiracy nut," he adds. He's not convinced his own version of events (minus the steamy videotape and adulterous couple) is correct, but he does believe many questions went unanswered in the official conclusion. He's ready to let readers decide.