Serial, the hit podcast that started the true crime trend in the format, will focus on the shortcomings and failures of the US criminal justice system in its third series by following individual cases in the courts of Cleveland, Ohio.

In a trailer for the new season, which debuts on 20 September, the show’s host, Sarah Koenig, explained how she had spent a year recording and following stories in local courts to build up a profile of their failings.

The first case she covered featured someone accused of mugging people at gun point. Although there was compelling evidence suggesting his guilt, Koening said that the process was hugely flawed.

“I’d seen a litany of things that shouldn’t be allowed,” she said. “It was like a checklist of things reformers complain about: extra charges loaded on to a case, pressure to plead, shabby police work …”

Koenig and the Serial team will follow “ordinary cases” and show how the impact of decisions made in the courts reverberate in wider society.

Serial came to prominence in 2014 when it followed the case of Adnan Syed, who was convicted in 2000 of murdering his ex-girlfriend, Hae Min Lee, in Baltimore, Maryland. Koenig interviewed him, reviewed evidence from the case and tried to piece together what had happened.

The podcast inspired an online sleuthing campaign and renewed interest in the case. Syed is currently awaiting a new trial.

The second season focused on Bowe Bergdahl, the US soldier who was given a dishonourable discharge after walking off his base in Afghanistan and being captured by the Taliban, by whom he was held for five years.

The first two seasons have been downloaded more than 340m times.