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Sean Ellis will not be re-tried in Boston Police Det. John Mulligan's 1993 murder, Suffolk County DA John Pappas announced.
Ellis was imprisoned for more than 22 years until the state SJC overturned his conviction, citing corruption of involved police detectives.
Pappas said his office decided against a re-trial after the "strength" of evidence "declined with time."
Suffolk District Attorney John Pappas announced Monday that his office is ending prosecution of Sean Ellis for first degree murder and armed robbery in the 1993 slaying of a Boston police officer.
The case is being dropped because prosecutors don't believe they would be successful in a retrial due to the amount of time that has since passed.
"The nature of the evidence has not changed in 25 years, but the strength of it has declined with time," Pappas said.
He also said the involvement of corrupt police detectives in the investigation has compromised their ability to prosecute the case.
"This was not an easy decision. It may not be a popular decision, but it is the right decision," Pappas said.
Ellis was sentenced to life in prison after he was convicted in 1995 of the shooting death of Detective John Mulligan. Mulligan was shot five times in the face at about 3:30 a.m. on Sept. 26, 1993, while he slept in his car while on a security detail outside a pharmacy.
The Supreme Judicial Court released Ellis from jail in 2015, saying he didn't get a fair trial because two police detectives who played key roles in the investigation later pleaded guilty to corruption charges, and prosecutors didn't provide Ellis' defense team with all of the evidence.
"It's kind of like surreal," Ellis told NBC10 Boston on Monday. "To say that I'm happy and ecstatic is an understatement."
He said he still had the GPS monitoring device on his ankle as he heard the news Monday.
Asked what he has learned about himself since 1993, he said he has learned that he is "resilient." He still maintains his innocence.
As for what's next, he said he wants to go back to school so he can get a better job. He's in his 40s now and says he's doing entry level work that someone in their late teens, early 20s would typically be doing.
Boston Police Commissioner William Gross said he agrees with the district attorney's decision but said that doesn't mean that Ellis didn't kill Mulligan.
"He's a father, and a brother and a son. It was with heavy heart we had to discuss this with the family for this final outcome," Gross said of Mulligan.
"Sean Ellis is culpable," he added. "Not innocent at all."