When he was a young crime reporter for The Los Angeles Times, Michael Connelly admired the detectives who worked a case even after it had gone colder than yesterday’s coffee. The ones who were “undaunted and relentless,” as he recently put it, restless and patient at the same time. It’s the quality that makes Harry Bosch, the detective Connelly created, such a memorable character.

“A city that forgets its murder victims is a city lost,” a captain tells Bosch in one of Connelly’s novels. Real cops he came to know liked that saying so much, they turned it into a sign at the cold case unit at the Los Angeles Police Department headquarters. When the office moved a few years ago, they gave the sign to Connelly.

So when two of Connelly’s most trusted collaborators, the retired Los Angeles Police Department detectives Rick Jackson (partly the basis for Bosch) and Mitzi Roberts (the inspiration for Ballard), came to him early last year with the claim that an amateur sleuth in West Virginia had solved the most notorious murder in L.A. history, there was only one thing Connelly could do: work the case as if he were still a young reporter ravenous for the big scoop. That investigation resulted in “Killer in the Code,” a podcast whose second season returns on Wednesday with the first two episodes of six.

The 10-episode first season, which debuted last December, roiled the true crime community by arguing that a World War II veteran named Marvin Margolis was responsible for two of the most famous unsolved crimes in California history: the 1947 Los Angeles murder of Elizabeth Short, immortalized as the Black Dahlia, and the five killings committed by the Zodiac throughout the San Francisco Bay Area in the late 1960s. “I have no doubts,” Connelly said. The evidence was “completely convincing.”