As she examines in excruciating detail the case against Mr. Syed, listeners are privy to her frustrations, doubts and random thoughts about the case. She sighs at times, tosses expletives and shares her own confusion. Recorded conversations with key players come across as casual chats rather than interviews. Listening to her is like hearing a riveting story from a close friend over drinks, a remarkable illusion that Ms. Koenig says took even her by surprise when fans of the series started emailing and calling her at home.

“There’s this thing where they assume we’re friends,” she said recently. “People feel like they know me.”

As the series progresses, listeners become deeply invested in the question that Ms. Koenig sets out to answer: whether Mr. Syed is innocent. I spoke to some people who found the series exploitive because it opened old wounds and turned private lives into the subject of global, frenzied speculation.

But I felt that any such effect was offset by how powerfully the series illuminated the complexities of the criminal justice system. In offering up the possibility that Mr. Syed may have been wrongly convicted, Ms. Koenig does not portray the police officers, the prosecutors or the defense attorney in a heavy-handed way. The subjects, including Mr. Syed, are vividly fleshed out and memorable. For the most part, they all seem to get to tell their side of the story.

Last week, a Maryland appeals court agreed to hold a hearing to explore whether Mr. Syed’s trial lawyer was ineffective, an effort that was in motion before the series began but may be bolstered by some of the facts Ms. Koenig uncovered. A separate team of lawyers who specialize in wrongful convictions reviewed the case after learning about it from the program’s producers and hopes to persuade a Maryland judge to order that DNA evidence from the case be tested, in case it points to a different suspect.

While both efforts are long shots, if a new trial does lead to an acquittal, “Serial” may have played a game-changing role in a case that was regarded as closed.

The series has set impossibly high expectations for a second season, which is expected to begin this fall. It is the most downloaded podcast on iTunes, with roughly 5.7 million listeners per episode. By comparison, the average download rate for the next 20 most popular podcasts is 446,000 per episode, according to Mark McCrery, the head of Podtrac, which monitors podcast traffic.

“No one thought ‘Serial’ would be as popular,” he said. “Bringing mysteries to podcasts is not something that has been done before in this way. But mysteries are very popular in other media.”