It sounds like a conspiracy: A week before a new book about a 40-year-old mystery comes out, someone turns up with fresh evidence that splashes the case all over TV and gives the author priceless publicity.

Coincidence? Well . . . yeah, probably. But that’s typical for events surrounding the disappearance of airline hijacker D.B. Cooper, a case at the intersection of real life and weirdness.

Geoffrey Gray couldn’t have asked for better advertising than the emergence two weeks ago of an Oklahoma woman named Marla Cooper, who got the FBI interested in her late uncle as a suspect in the nation’s only unsolved hijacking. Gray’s book on the case, “Skyjack,” hit the stands Tuesday. And while Marla Cooper’s uncle was not one of the characters Gray encountered in his quest for the elusive fugitive, he finds plenty of other plausible suspects in this cockeyed tale.

It all goes back to the stormy night of Nov. 24, 1971, when a man using the name Dan Cooper boarded a plane in Portland, Ore., bound for Seattle. Just after takeoff, he handed a note to a stewardess, asking her to sit by him and saying he had a bomb. He chatted politely, then demanded $200,000 and parachutes.

In those days, hijackings were practically a fad. Seemingly any plane trip could wind up with an unscheduled stop in Cuba. So the flight crew and authorities on the ground were accommodating to the strange man, who was either tall or short, fair or dark, depending on who later recounted the incident. They landed in Seattle, got him the cash, cleared out the passengers and took off again.

‘SKYJACK: The Hunt for D.B. Cooper’ by Geoffrey Gray. GrayCrown. 3202 pp. $25 (Crown/Crown)

When the plane touched down in Reno, Nev., several hours later, Cooper and the money were gone. He had jumped out. In the 40 years since, no one has ever figured out who he really was.

I had assumed that anybody who lived through the 1970s would know the name D.B. Cooper, as the hijacker was mistakenly called in the press. He was a real-life Bigfoot, an instant folk legend in an era when outlaws seemed more appealing than the establishment goons who brought you Vietnam and Watergate. Cooper became the subject of popular songs, TV shows, movies — heck, there are still restaurants named after him.

So it was surprising when I surveyed friends and colleagues in their 40s and got a fair number of blank looks. But among people who spend a lot of time online — young or old — D.B. Cooper was instantly familiar. That’s because the legend continues to ferment and grow on the Internet.

In “Skyjack,” Gray dives into the world of online sleuths, speculators and zealots as if he were Hunter S. Thompson hitting the road with the Hell’s Angels. There’s a fair amount of gonzo in Gray’s telling of the tale, which ultimately becomes as much about obsession and the kookiness of human nature as about the case itself.

Even in his first encounter with a possible suspect, Gray feels his grip slipping. Here he digs through some of the man’s papers: “I flip back to Kenny’s military records. In the far corner of one form I see his thumbprint, taken May 25, 1944, when he enlisted. I place my own thumb on it. I close my eyes. I imagine what Kenny’s hands must have felt like. I imagine my own thumbprint on his — to summon a feeling, an out-of-body clue that would tell me if Lyle’s brother was indeed D.B. Cooper.

“I feel something. Really. I swear. Kenny?

“Or do I feel anything? Am I trying too hard and making it all up?”

And he’s just warming up.

Gray, a contributing editor at New York magazine, takes an interest in the story after hearing a former street preacher turned private eye spin a wild tale in a New York bar. Gray heads out West, thinking he’s on to the solution of the case. Visions of Pulitzer Prizes dance in his head. While that first tip doesn’t quite pan out, it leads him to another tantalizing possibility. And then he’s hooked.

The truth is out there, as Agent Mulder used to say. And in this case, it really is. Either Cooper’s body plopped down and decomposed in the wilderness, or he hit the ground running and lived the rest of his life with a nice bit of cash. Somebody knew him, before or after. He is, undoubtedly, findable. Gray unearths clue after clue, and even the wildest theories turn out to be maddeningly plausible. Including the one about how Cooper may really have been a woman.

“Skyjack” toggles back and forth between two narratives — a reconstruction of the original case and manhunt, and the author’s descent into the heart of obsessive darkness. He spends a considerable amount of space evoking that long-ago era — smoking on planes, the Vietnam War’s mounting toll, rioting in cities — and while at first he seems to be padding the story, all the context serves a purpose. It helps explain why someone would do what Cooper did and why he became a hero. And just maybe, it hints at something bigger behind the case itself. Yes, a conspiracy.

Gray digs up a mountain of interesting detail. He got access — apparently for the first time — to the FBI’s original investigative files, and he interviewed the surviving principal players.

But it’s his journey into the lives of Cooper suspects and fanatics that eventually takes over the book. The damaged people who might have had a motive. The family members who always wondered. And above all, the investigators and busybodies who have dogged the case for years and years, searching for truth.

What is it about mystery, this one or any other? Some religious impulse, surely, that compels people to seek signs of the extraordinary beyond the realm of everyday life. When the Internet theorists who worship the Cooper case finally get together in the flesh, they face the inevitable disappointment of reality, bickering and feuding and cooking up fevered rumors about one another’s motives. Gray falls victim to this, and while his inside look at fanaticism seems to be one of the points of “Skyjack,” he begins to lose control of the narrative. It’s a bit frustrating that a book that starts out promising to give answers winds up being so messy and personal.

Toward the end, the author is in mental squalor, pawing through the credits of a “post-apocalyptic” porno film, looking for a name. He finds it, which sets off a cascade of speculation: “Wasn’t that Duane Weber’s friend? Didn’t Jo Weber claim to meet Gunn once in Mobile, Alabama? Didn’t he tell her to burn all her files and that Duane knew people in high places? Is Gunn McCoy? Is McCoy still alive?”

That passage doesn’t make much more sense, even if you know who those characters are. There may be answers, somewhere. Or maybe the FBI will pin the whole thing on Marla Cooper’s uncle. But the one thing “Skyjack” makes entertainingly clear is that it’s a weird, weird world.

Greg Schneider is The Washington Post’s national economy and business editor.

Book review by Greg Schneider