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It's one of the most popular podcasts ever, and amateur detectives are trying to get to the bottom of the true-life murder at the heart of it. But how is social media sleuthing influencing the case and the real people caught up in it?

Serial is a spin-off of the US public radio programme This American Life. (If you're already among its legions of devoted fans you can skip this paragraph.) For nine episodes it's been re-investigating the case of Hae Min Lee, a teenager who was murdered in Baltimore. Her ex-boyfriend Adnan Syed was convicted of the murder and has been in prison for the past 15 years. In a huge journalistic undertaking, Serial's producers, led by Sarah Koenig, have examined the case from nearly every angle.

And because it's a real-life crime, the series has inspired fans to start their own investigations. A section of reddit devoted to Serial has attracted more than half a million users, and fans have been hunting for clues both online and at key locations in and around Baltimore.

"I was hooked from the beginning," says one reddit moderator who goes by the handle "wtfsherlock". His real name is Mike and he lives in California, but he didn't want to reveal any more than that to BBC Trending.

Amateur detectives have dug up and shared links to the criminal records of witnesses, pointed out similar cases nearby and have even travelled to the Best Buy electronics store at the centre of the case, to try to track down evidence of a pay phone that a key phone calls allegedly came from.

Serial podcast stats

20 million total downloads; 2.23 million per episode

Fastest podcast ever to reach 5 million iTunes downloads

Top of the podcast charts in the US, UK, Germany, Australia and elsewhere.

Sources: Serial, Apple, iTunescharts.net

Mike says he's done some fact-checking himself. In an early episode, it was revealed that a potential witness might have been able to provide an alibi for Adnan Syed when she claimed to remember seeing him just before a snowstorm.

"I went and looked back at National Weather Service data, and it showed there was no snow on that day at all - it was actually quite warm," he says. "There was a storm, but it was the next day."

The weather discrepancy was later noted in a Serial blog post.

The producers of Serial, citing deadline pressure, said they didn't have time to answer our questions about how social media has affected the course of the series. In a New York Times interview, Sarah Koenig said she was surprised at the popularity of the podcast and denied that feedback was guiding the future of the series. But she also said producers were keeping one eye on the online conversation.

Image copyright Reddit Image caption Reddit has a section devoted to Serial

"I'm not looking at reddit. But we do have people on the staff who are dipping into it to make sure there's nothing crazy on there or something we're missing - they might know something we don't."

But Mike, aka wtfsherlock, thinks the social media discussion has influenced the course of the series. For example, one of the recent discussion points on reddit and elsewhere has been about the lack of comment by Hae Min Lee's family. Episode nine then included a passage in which Sarah Koenig detailed the lengths the podcast's producers have gone to find family members, including hiring two private detectives and Korean-speaking researchers.

Image copyright AP Image caption Adnan Syed

"In my 20 plus years of reporting, I've never tried so hard to find anyone," Koenig said of Lee's mother. "I learned a few days ago that they [the Lee family] know what we're doing. My best guess is they want no part of it, which I respect."

Mike says reddit moderators have had some minimal contact with the producers and that he understands why they haven't leaned more on reddit for research tips.

"They are very much doing their own thing," he says. "I think it would be very risky for Serial to say, 'could you find out where so-and-so lives?' Because people would do it. And that would be problematic."

Mike says that moderators do their best to delete personal information leaked on the forum - although with so many posts many details have been leaked online - and the subreddit rules clearly ban posting such information about any of the characters mentioned in the podcast.

"It's a weird situation because reddit's policy is 'no personal information'," he says. "Well here we have a real-life story, with real-life names, that are being broadcast by podcast to millions of people every week.

"Our policy has been to keep people from revealing things [about individuals] that aren't revealed publicly by the podcast, and particularly keeping last names out of the discussion."

But Jim Trainum, a former detective hired by Serial to look at the case, told BBC Trending that online speculation could have serious implications for the people involved in the case.

"Because of this frenzy, it does impact lives adversely," he says. "Jumping from A to Z with no steps in between, making assumptions based on gut reaction … too often a rumour takes on a life of its own and soon becomes a fact."

The activity on reddit is so frenetic that it even attracted someone claiming to be Hae's brother, who wrote an emotional plea asking people to leave the family alone.

If the podcast producers are concerned about the privacy of the Lee family and the podcast's interviewees, they have more than social media to worry about.

Personal details including Facebook pictures of key witness Jay - Serial producers scrupulously refer to some characters by first name only - have been posted on the Daily Mail website (and no, we're not going to link to it).

As for Adnan Syed, a court is currently reconsidering his case. That appeal, which started in September and is centred around whether he received adequate legal representation at his original trial, has little to do (so far) with the drama unfolding on Serial.

The next edition of the podcast will be available Thursday - but if you're a listener, you knew that already.

Blog by Mike Wendling, Ruth Alexander and Gemma Newby

Reporting and graphic by Ravin Sampat

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