On Tuesday, Sean Ellis walked into a room inside the Suffolk Superior courthouse, and the electronic bracelet he had worn around his ankle since 2015 was removed.

He said it was the first time he felt free.

On Monday, Acting Suffolk County DA John Pappas announced that his office would not retry Ellis for the 1993 murder of detective John Mulligan.

In 1995, Ellis, at the time 19 years old — along with then 18-year-old Terry Patterson — was convicted of first-degree murder and armed robbery. Ellis spent 22 years behind bars.

Pappas told reporters this week that the involvement of three corrupt officers and the passage of time “compromised” the case, even though he and Boston Police Commissioner William Gross said they believe that Ellis committed the crime.

“Jurors at the time called the case against him overwhelming," Pappas said, "but the passage of more than two and a half decades has seriously compromised our ability to prove it again.”

Mulligan was murdered 25 years ago at the entrance to a Walgreens store.

In the early morning hours before dawn, as Mulligan sat in his car on a paid security detail, someone crept up to his car and shot him five times in the head.

Walgreens was open 24-hours at that time, and people came and went. Ellis and Patterson, later convicted of Mulligan’s murder, told a court they were there to buy pampers for Ellis’ cousin, who sat in the back of the car while the two men went inside.

An eyewitness, Rosa Sanchez, identified Patterson and Ellis as the men hovering near Mulligan's car. Two trials ended in hung juries. Then, after a third trial, both men were convicted, with evidence collected by investigating detectives Kenneth Acerra, Walter Robinson and John Brazil.

But hidden from the juries was key information showing that all three cops were corrupt. Also not known at the time was that Mulligan, Robinson, Brazil and Acerra — according to testimony before a grand jury in 2013 — had robbed a drug dealer on Commonwealth Avenue in Boston, a few weeks before Mulligan was murdered. It was one of many incidents of corruption that never made it into the case file during discovery. Acerra and Robinson were convicted of perjury and armed robbery in 1998 in an unrelated case. Brazil admitted to a lesser charge.

Sanchez, the key eyewitness against Ellis and Patterson, was also related to one of the investigating corrupt officers. This and evidence of law enforcement corruption, kept under wraps by the Boston Police Department and the Suffolk County DA, were among several factors that prompted District Judge Carol Ball in 2015 to release Ellis on bail and to order a new trial. That decision was affirmed by the Supreme Judicial Court in 2016.

Ellis had always maintained his innocence, but that had no bearing on Ball’s decision. She was concerned that he had not received a fair trial.

That trial — which would have been the Ellis' fourth — was set to begin in September next year before Pappas announced that his office would not retry him.

Pappas also asserted that police corruption, although a factor, had not impacted the earlier trials.

“As the SJC concluded 18 years ago, there is no reliable evidence that Acerra, Robinson, or Brazil procured or produced false evidence in this case. Based on the facts and circumstances known to us, we don’t believe Detective Mulligan was involved in their schemes,” Pappas said.

But Boston area-researcher, Elaine Murphy, an advocate for Ellis, points to a federal grand jury testimony of drug dealer Robert Martin in 1998 who identified Mulligan, alongside Acerra and Robinson, as one of the detectives who robbed of two of his apartments. It was one of several robberies of drug dealers and pimps that witnesses said included Mulligan, according to a BPD Internal Affairs file that was released by court order in 2015.

Sean Ellis with his mother Mary "Jackie" Ellis, and his sister, Shar'Day Taylor in their home in Lynn, MA.
Phillip Martin, WGBH News

And Ellis’s attorney, Rosemary Scapicchio, said prosecutors failed to pursue other leads. “I would hope that the Suffolk County District Attorney’s Office would continue their investigation and actually find and prosecute the people who are responsible for Detective Mulligan’s murder,” she said.

Scapicchio said they were ecstatic that Pappas had dropped the charges but blasted prosecutors for insisting in a press release that her client was still guilty.

“We're a little annoyed at the way they did it because they're still saying Sean is guilty," she said. "They just can't prove it, and we don't think that that's true at all.”

Mulligan’s family remains convinced that Ellis was involved in his murder but is not keen to talk to reporters.

Ellis lives with his mother and sister in Lynn. He said the next step in his life is to apply to college and continue his work with the Innocence Project.

“I’m not the only one that’s gone through this horror,” he said. “Wrongful convictions are epidemic.”

Correction: An earlier version of this story incorrectly stated the year of the federal grand jury testimony of drug dealer Robert Martin. The testimony occurred in 1998.