After a night of soul-searching and meetings with executives and lawyers at HBO, they decided they had a moral obligation to turn the knife over to the authorities, even though they worried that doing so might compromise their relationships in the community and hurt their film. “We felt it was incumbent on the filmmakers to make sure that that information was in the hands of the authorities as early as possible,” Mr. Berlinger said.

(HBO was not involved in the discussions over when to approach law enforcement about Mr. Durst. The film was already almost finished when the cable channel acquired it in the fall.)

Mr. Berlinger described “The Jinx” as a “triumph,” but he also said documentarians were under increasing pressure from networks to make films that more closely hewed to the conventions of scripted TV shows, like procedural crime dramas, which withheld information from the viewers until the very end for maximum impact.

“Real life doesn’t necessarily mirror the arc of scripted drama, and yet there has been this push in television to bring those two kinds of storytelling together,” he said. “That doesn’t make it wrong or bad. It just makes it a morally interesting time for documentary film.”

Mr. Durst’s bathroom mutterings were the film’s biggest bombshell, but they may be of little use when his case goes to trial. To begin with, the prosecution will need to prove beyond a reasonable doubt that the recording had not been tampered with. The defense can also argue that it came after the filmmakers had ambushed Mr. Durst in an interview, and that he was rattled and not thinking clearly.

The formulation of the apparent confession was problematic in its own right. It was suggestive, but by no means definitive. In a column on Bloomberg View, the Harvard Law professor Noah Feldman compared it to a Shakespearean soliloquy. “Even the question-and-answer form (‘What the hell did I do? Killed them all, of course’) is reminiscent of the untrustworthy soliloquies delivered by Hamlet,” Mr. Feldman wrote. “The soliloquist asks himself the big questions while alone on stage (‘To be or not to be?’), and tries on different answers.”

The timing of Mr. Durst’s arrest may have seemed choreographed, but it is also true that as “The Jinx” moved toward its climax, the odds seemed to grow that he might try to flee. When he was taken into custody while staying in a New Orleans hotel under a false name, the makers of “The Jinx” were relieved. “We were concerned that Bob was floating around,” said Mr. Jarecki, noting that he and Mr. Smerling had begun to fear for their safety.