When he finally emerged from the Los Angeles field office, Mr. Broeksmit got into a Lyft and called me. His adrenaline, I could tell, was still pumping; he was talking so fast he had to stop to catch his breath.

“I am more emotionally invested in this than anyone in the world,” he said. “I would love to be their special informer.”

The next great American whistle-blower

Here’s the thing about whistle-blowers: They tend to be flawed messengers. Daniel Ellsberg, Chelsea Manning, Edward Snowden — each of them was dismissed as selfish, damaged, reckless and crazy. Yet all of them, regardless of motivation, used secret documents to change the course of history.

For more than five years, Val Broeksmit has been dangling his Deutsche Bank files in front of journalists and government investigators, dreaming of becoming the next great American whistle-blower. He wants to expose what he sees as corporate wrongdoing, give some meaning to his father’s death — and maybe get famous along the way. Inside newsrooms and investigative bodies around the world, Mr. Broeksmit’s documents have become something of an open secret, and so are the psychological strings that come attached. I pulled them more than anyone, as part of my reporting on Deutsche Bank for The New York Times and for a book, “Dark Towers,” to be published next year. It has been the most intense source relationship of my career.

An endless procession of bank executives and friends of the Broeksmit family have warned me that Mr. Broeksmit is not to be trusted, and, well, they might have a point. His drug use has sent him reeling between manias and stupors. He has a maddening habit of leaping to outrageous conclusions and then bending facts to fit far-fetched theories. He fantasizes about seeing his story told by Hollywood, and I sometimes wonder whether he’s manipulating me to achieve that ambition. He can be impatient, erratic and abusive. A few days ago, irate that he was not named in a blurb for my book on Amazon, among other perceived slights, he sent me a string of texts claiming that he’d taken out a brokerage account in my name and traded on secret information I’d supposedly fed him. (This is not true.) A little later, he left me a voice mail message saying it was all a joke.

Why do I put up with this? Because his trove of corporate emails, financial materials, boardroom presentations and legal reports is credible — even if he is not. (In this article, every detail not directly attributed to Mr. Broeksmit has been corroborated by documents, recordings or an independent source.) Besides, there’s something uncanny about how Mr. Broeksmit’s fearlessness and addiction to drama have led him, again and again, to the center of the news. In addition to Deutsche Bank’s troubles, he has figured into North Korea’s hack of Sony Pictures, the collapse of the world’s oldest bank and the House Intelligence Committee’s ongoing investigation into Mr. Trump.