While developing material for a third season of the landmark podcast Serial, its host and executive producer Sarah Koenig spent a year inside the Cleveland, Ohio, criminal courthouse, looking for the next big miscarriage of justice.

But in the end, she did what any good reporter would do: stop looking for something that’s not there; focus on what’s in front of her; and report that. The result of Koenig’s work will be revealed when the podcast’s new season kicks off on September 20.

Serial burst onto the scene in 2014 when it told the story of the trial of Adnan Syed and his conviction for the murder of fellow high-school student and ex-girlfriend Hae Min Lee. It was riveting listening – and an episode’s running time meant that at least one day a week alternative, longer, public transport routes were taken so an entire episode could be heard in one sitting. And then there were the hours examining the additional material posted on the Serial website to provide greater depth and background.

The podcast was not just an exploration of the US justice system and how it worked (or perhaps did not work) in a particular case, because that on its own would not have been enough to establish Serial as a must-listen. To a greater extent, Serial examined fundamental questions of identity and character, of relationships and loyalties, and of prejudices and perceptions.

The first series of Serial has reportedly been downloaded more than 175 million times, and as a result Syed, now 36, had his original conviction set aside, and in March this year was granted a second trial, with the Maryland Court of Special Appeals saying Syed had received “ineffective assistance of counsel” at his trial, because his lawyer had failed to call a witness who, if their testimony were to be believed, “would have made it impossible for Syed to have murdered Hae”. (Ongoing legal issues mean the timing and the grounds for a new trial currently remain uncertain.)

Serial returned in 2015 for a second series that examined the case of Bowe Bergdahl, a US soldier captured by the Taliban and held captive for five years, before being released and returning to the US to an expected hero’s reception. That is, until a quite different story began to emerge from Bergdahl’s own unit, leading him to being charged with desertion, and facing a potential life sentence.

Unsurprisingly, there was more to Bergdahl’s story than met the eye – or media coverage – but perhaps inevitably, Serial’s “difficult second album” failed to scale the heights of its predecessor.

In the preview to series three, Koenig says she recognised early on from watching trials in the Cleveland courthouse that a better understanding of the criminal justice system could be gleaned from the deadly ordinary cases that seem to clog up the system, not the extraordinary ones, like Syed’s and Bergdahl’s.

“We can’t understand how the criminal justice system works by interrogating one extraordinary case,” Koenig says. “Ordinary cases are where we need to look.”

She recalls watching a trial in the Cleveland courthouse which encapsulated “a litany of things that shouldn’t be allowed”.

“It was like a checklist of almost all the stuff reformers complain about: extra charges loaded onto a case; pressure to plead; shabby policework; a police officer possibly lying on the stand; eleventh-hour evidence shoehorned into a trial; overworked attorneys; dozing jurors; a dozing judge; and finally, an outsized prison sentence,” Koenig says.

“But on one was stomping their feet too hard about that. In the Cleveland courthouse this was a nothing case. No one was really watching. And besides, why would anyone care? [The defendant] was guilty.”

So time Koenig and the Serial team are taking a different tack, abandoning the format of following a single case and instead examining “the entire criminal justice system”, across multiple cases, and over the course of a year.

“We chose Cleveland because they let us record – everywhere,” Koenig says.

“And we followed the cases outside the building, into neighbourhoods and people’s houses and into prison. In every case we looked into there came a time when I said, ‘Wait, this can’t be happening’. And then it did.”

What: Serial, a podcast from the creators of This American Life, produced in collaboration with WBEZ Chicago

When: Starting September 20

Where: Serial website, Apple Podcasts, Google Podcasts, Pandora, Stitcher, RadioPublic (learn more about podcasts.)

How to get the most out of the Serial podcast:
Subscribe to receive new episodes as they’re released
Check the Serial website regularly to see additional material and documents
Find an online forum – like Hunter & Bligh – to debate and discuss the issues as they arise