For 15 years, nobody outside Maryland cared much about the murder of a South Korean-born high school teen, supposedly at the hands of her ex-boyfriend, the son of Pakistani immigrants.

Now, it seems, everyone does.

The perplexing tale of Hae Min Lee and Adnan Syed is at the heart of "Serial," an hour-long weekly podcast that's become an unlikely global Internet phenomenon.

Fans speak of being "addicted" and "obsessed" with the program.Those who caught the bug early can't wait for Thursdays, when fresh instalments drop. Latecomers binge on past episodes.

It's been downloaded more than five million times from Apple's iTunes store, where it's a Top 10 hit in the United States, Canada, Britain, Australia, India, South Africa and Germany.

Who's telling the truth

It can also be heard on the show's www.serialpodcast.org website.

Between episodes, online chatter rages on social media. Reddit hosts an exhaustive "Serial" discussion board. Bloggers speculate who's telling the truth -- and who might not be.

"Serial" is a spin-off from "This American Life," a long-running and hip US public radio series that's famous for quirky topics and laid-back story-telling style.

Its runaway success -- as a podcast, no less -- has taken its creators by surprise.

"We kind of expected to be in the sleepier realms of the podcast world," senior producer Julie Snyder told AFP on Tuesday in a telephone interview from New York.

We were hoping for good numbers. But we were not at all expecting so many people listening and writing about the show and having a lot of interest about the show," she said.

"And it's international. We didn't plan for that at all."

Serial" comes across as part investigative journalism, part police procedural, part soap opera, with a nod to the 19th century serialized novels of Charles Dickens and Emile Zola.

Hae and Adnan -- everyone in "Serial" is called by their first name by narrator and journalist Sarah Koenig -- are high school sweethearts who keep their love a secret from their conservative immigrant families.

In the opening episodes, both come across as bright all-American teenagers -- popular, getting good grades, holding down part-time jobs, looking forward to prom night.

But when romance turns to break-up, Adnan, overcome by anger, strangles Hae and, with a pal, buries her in a shallow grave, where a passerby finds her three weeks later.

At least, that's the version that prosecutors gave jurors at a six-week trial that ended with Adnan getting a life sentence in a Maryland penitentiary, where he remains at the age of 32.

Koenig revisits the case in forensic detail -- interviewing witnesses who sometimes contradict themselves, pursuing neglected leads, chatting regularly by phone with the imprisoned Adnan, who maintains he is innocent.

Looking for a fix

One "Serial" fan is Emily Best, an indie film crowdfunding consultant, who binged-listened to the first seven episodes while driving non-stop across the entire state of Kansas.

When Thursday came around with episode eight, she told AFP by email, "we were up early in the morning like junkies looking for a fix."

In California, teacher Michael Godsey revealed this week that he is using "Serial" in lieu of Shakespeare in his high school English class this semester.

"In fact, it’s been more fun, more engaging, and more conducive to learning ... than anything written by Shakespeare, Joyce, or anybody else," he wrote on his blog. "By far."

Unclear is whether "Serial" might turn up fresh material that would compel judicial authorities to reopen the case -- and Snyder said that's not the point of the show, either.

"We've said from the beginning that we don't know where it's going to end," she added, ahead of Thursday's release of episode nine.

"Serial" is likely to run for about 12 episodes overall, but the producer cautioned: "We don't know for sure, because we are still doing the reporting."