Nearly 30 years ago, Sean Ellis, a young Black man, was accused of killing white Boston police detective, John Mulligan. Ellis was wrongfully convicted and spent more than 20 years in prison. He's now a free man, and the subject of a Netflix docuseries, Trial 4. GBH News reporter Phillip Martin has been covering Ellis for years and is featured in Trial 4. He discussed the case and where it stands with GBH All Things Considered host Arun Rath. This transcript has been edited for clarity.

Arun Rath: I think that people who are old enough will probably remember this very well, even though it was back in 1993. But for those who might not know, tell us the basic facts of this case, this officer who was killed. It was pretty horrific.

Phillip Martin: It was shocking. It was in the early morning hours in September 1993, the call was basically that a police officer had been shot sitting in his car outside a Walgreens. As you can imagine, dozens of police cars came on the scene, and that's where they found John Mulligan, shot five times.

The only way you could describe it is as an execution-style murder. Mulligan made a lot of people angry on the streets. He was flamboyant, and he was brutal to those he deemed as criminal. Quite often those he deemed as criminal were young African Americans, Latinos, and oftentimes young women who he had a reputation for victimizing. So he had a lot of enemies. No one was sure who could have done this, but it was clear that it was someone who did not like John Mulligan. This was personal.

Rath: So how did it come to be the case that Sean Ellis ended up being charged in this horrible crime?

Martin: According to his defense attorney, Rosemary Scapicchio, Ellis had gone to this Walgreens at about three o'clock in the morning to buy Pampers for his cousin, who he lived with. He was in the company of another young man, and they were basically implicated in the case. Authorities said that he wasn't just there to buy Pampers, as Ellis had said, but that he actually had carried out this murder.

Now they were trying to find a motive. Why would he carry out this murder? His friend, Terry Patterson, it was alleged, according to police, had basically wanted to steal the officer's gun. They claimed that Patterson had killed Detective Mulligan and had engaged his friend Sean Ellis, and together this joint enterprise of murder had taken place. Over the course of this docuseries, they show that stealing the gun was implausible. There were a lot of things about this case that make absolutely no sense, and the docuseries does better than most in showing why it's problematic.

Rath: There's obviously a lot of detail, because this is an eight-part docuseries. But in shorthand, there are a lot of issues around police corruption. Could you speak to that point, of what policing was like in Boston circa 1993?

Martin: That's right. At that time, you had several police officers involved in this case, Kenneth Acerra, Walter Robinson and John Brazil. Two of these officers, Acerra and Robinson, were later sent to prison for corruption. It was later determined that they were working in collaboration with John Mulligan. He too was corrupt.

So you had a police department in 1993 that was riven with corruption. The police department says they've done a great job since then of cleaning up a lot of that corruption, and that's no doubt true. But problems still remain within the Boston Police Department that are still unaddressed, including who actually shot John Mulligan, and why was this murder attributed to Sean Ellis with no examination of some of the discovery that was uncovered by Rosemary Scapicchio and others that pointed to others who could have been involved in this case? And that also explains, and this may lead to the next question, of why there was no new trial.

Rath: Right, because after the charges were thrown out and Ellis was freed. He was supposed to have another trial, right?

Martin: That's right, and that trial would have been around now. What made the difference was a new district attorney. The former Suffolk County DA, Dan Conley, was intent on trying Sean Ellis again. But Conley quit and started working for a law firm. Rachael Rollins ran for the district attorney's office and won. She started to look at the Sean Ellis case, and she saw a lot of things that are problematic, including some of the things I've described here. Before she took office, the acting Suffolk County DA John Pappas said they would not pursue a fourth trial for Sean Ellis. For Sean Ellis, that was both good and bad. It's good that he did not have to remain in jail, but the bad, according to him and his attorney, is that he did not have an opportunity to exonerate himself.

Pappas and Boston Police Commissioner William Gross, even though they decided not to pursue the case to a fourth trial, said that they felt that beyond question, Ellis is guilty. Now, Sean Ellis felt and his lawyer felt that if a fourth trial had occurred, they could have shown the corruption in the police department, and they could have shown and implicated John Mulligan in that corruption. They felt they could have shown that the gun, which was allegedly taken by Sean Ellis, was not taken by Sean Ellis, and they felt that was a setup, eventually resulting in his complete exoneration rather than having this case still hanging over his head. There's still a gun charge on his record, he's still charged with having taken this gun from Detective Mulligan. What his attorney would like to do right now is to have that charge dismissed.

Rath: Before we let you go, I've got to ask you about about you, because you're in this docuseries. When do we see Phillip Martin, and what role are you playing in this documentary?

Martin: If it it were a movie, it would be a supporting role or a cameo. But as it is, I largely appear in these docuseries at the time that DA Rollins is running a very extraordinary race between several candidates of very different personalities, where folks believe, particularly the American Civil Liberties Union, that a DA can make all the difference in the world in terms of how justice is meted out and even defined. So my role is basically covering the race for district attorney and taking a deeper dive into the Ellis case and basically trying to ask questions. So the role I play largely is that of the role I play every day, which is that of reporter, investigator and analyst.