BOSTON — It was a gruesome killing with a sympathetic victim: a veteran Boston police detective, shot in the face five times as he slept in his car while on security detail outside an all-night pharmacy.

Nineteen-year-old Sean Ellis was one of two men convicted of killing Detective John Mulligan to steal his gun as a “trophy.”

Two decades later, Ellis’ attorney, Rosemary Scapicchio, says she has new evidence that corrupt police officers covered up potential suspects in the crime, including a fellow police officer. She asked a judge to order a new trial for Ellis; a hearing has been scheduled for next month.

“He’s served 21 years for a crime he didn’t commit,” Scapicchio said.

Prosecutors, however, say the jury that convicted Ellis of first-degree murder had more than enough evidence, including statements from witnesses who said they saw Ellis crouching near Mulligan’s car and then speeding away in another car.

“There is not one aspect of this motion that sheds any doubt on the strong evidence that proved his guilt at trial,” said Jake Wark, a spokesman for Suffolk District Attorney Daniel Conley.

Ellis stood trial three times before a jury convicted him in Mulligan’s slaying. The first two trials ended with hung juries.

Ellis’ co-defendant, Terry Patterson, was found guilty in a separate trial, but his conviction was later overturned. He ended up pleading guilty to manslaughter and was released from prison in 2007.

During their trials, Ellis and Patterson each accused the other of killing Mulligan.

Ellis has insisted he went to Walgreens in Boston’s Roslindale neighborhood early on Sept. 26, 1993, after his cousin requested diapers for her baby. He said he flagged down Patterson, a friend, for a ride.

One of the main allegations in the motion is that prosecutors failed to disclose that the Boston police knew Mulligan was part of a group of officers who were robbing drug dealers and other criminals.

Ellis’ attorney alleges that three weeks before he was killed, Mulligan was involved with other officers in stealing thousands of dollars from a drug dealer under the guise of a drug arrest. Based on the dealer’s allegations and other evidence, two police officers were convicted of corruption charges.

“It’s easy to make allegations about a dead man’s character and actions many years ago,” Wark said. “It’s much harder to dispute the evidence that proves Sean Ellis’ guilt.”

Scapicchio said the newly obtained evidence, contained in FBI reports, “creates a strong reasonable doubt about whether Mulligan was killed not by Ellis, but instead because of his alleged involvement in the criminal conspiracy” with other officers.

Scapicchio said FBI reports indicate that police knew that an officer had accused another officer of killing Mulligan. She also argues that the drug dealers who were robbed could have killed him.

Prosecutors dismiss the defense claims.

“This latest motion for a new trial requires us to believe that a mystery man murdered Detective Mulligan right after Sean Ellis was seen creeping around his car and right before Sean Ellis was seen speeding away from the scene,” Wark said.

Mulligan’s younger brother, Richard Mulligan, said Ellis is trying to win his freedom by besmirching his brother’s name.

“John to me was a hero,” he said. “He wasn’t crawling around in the woods on a rainy Sunday morning to shoot someone in the face to steal his gun. Not everyone is a criminal because Sean doesn’t like being in prison anymore.”