The new Netflix docuseries, "Trial 4," explores the case of Sean Ellis — a Black man from Boston who spent 22 years in prison for the murder of a Boston police detective. It was a murder Ellis always maintained he did not commit. His conviction was ultimately overturned and the charges against him were dropped, but he never got the chance he hoped for, to prove his innocence in court.

“There’s a problem in this country,” Ellis told Jim Braude on GBH News’ Greater Boston Monday, when asked why he decided to take part in the documentary. “When I think about what is going on with Black and Brown people within this criminal justice system – with mass incarceration, police brutality, things of that nature … there needed to be something that shed light on the injustices.”

Ellis's first two trials ended in a hung jury — although one did convict him of weapons possession — and in his third, he was convicted of first-degree murder. He was freed on bail in 2015, after a judge determined he did not get a fair trial, but he faced the prospect of a potential fourth trial until Acting District Attorney John Pappas announced in December 2018 that the charges against him were being dropped.

The numerous problems with the investigation and prosecution of the case are explored in the eight-part series, now on Netflix. Detective John Mulligan and three other detectives had been accused of robbing a drug dealer just 17 days before Mulligan was shot in the face and killed while in his car outside a Walgreens, where he was working a private security detail in 1993.

Those three detectives played key roles in the murder investigation, but two later pled guilty to corruption charges and a third was granted immunity for his testimony against the other two — points that Pappas maintained were unrelated to Ellis' case during his 2018 announcement.

But in the 2015 ruling that overturned the conviction, Judge Carol Ball said that Boston police received tips from three different individuals after the shooting — which pointed to a police officer and his son as possible suspects in Mulligan’s murder — and that Ellis’ defense team had not been made aware of this information.

Even the process of trying to get people to speak for the documentary was instructive, according to filmmaker Remy Burkel. While some agreed to go on camera, he said, “other people just said, ‘No way, not in Boston. I’m not gonna go on camera, those police officers are still out there. We’re still afraid of blowback.’”

Still, through everything, Ellis always maintained his innocence.

As for the question of whether they know who did murder Mulligan, Ellis' defense attorney Rosemany Scapicchio told Greater Boston, “I do not. But I know no one’s looking for them. So that’s a problem.”

Scapicchio is also pushing current District Attorney Rachael Rollins to drop one final remaining conviction.

“I expect that we’re going to continue to talk about trying to get rid of Sean Ellis’ gun conviction, because he didn’t do it, and he shouldn’t be a convicted felon because of this case,” she said.

As for the possibility of full exoneration?

“I would love to see Sean exonerated," Scapicchio said. "That would be a dream come true to him and to me.”