The John Mulligan Murder Case — Boston Detective Killing

The Case

The John Mulligan murder case is one of the most contested police-killing investigations in modern Massachusetts history. On September 26, 1993, veteran Boston Police Detective John J. Mulligan was shot five times in the face while sitting in his vehicle on a paid security detail outside a Walgreens drugstore in Roslindale. Two young Black men from Dorchester — Sean K. Ellis and Terry Patterson — were arrested, prosecuted, and convicted, but both convictions later collapsed amid evidence of investigative misconduct and unreliable forensic science. Today, more than three decades after the killing, no one stands legally convicted of murdering Detective Mulligan.

The Crime

In the early hours of September 26, 1993, Detective Mulligan was working a private security detail in the parking lot of a Walgreens at 972 American Legion Highway in the Roslindale neighborhood of Boston. He was sitting in his Ford Explorer, reportedly asleep, when an assailant or assailants approached the driver's side window and shot him five times in the face — an attack described by multiple sources as execution-style.

A Walgreens employee returning from a break discovered Mulligan slumped in his SUV at approximately 3:49 a.m. Investigators concluded that the motive was the theft of Mulligan's service weapon, which was missing from the scene.

The Victim

John James Mulligan was a 52-year-old detective with the Boston Police Department who had served the city for 27 years. According to his memorial on the Officer Down Memorial Page, he was on a secondary employment detail — a common arrangement in which Boston officers earned extra income guarding businesses during off-duty hours — when he was killed. He left behind family and colleagues in a department that would be shaken not only by his death but by the corruption allegations that later emerged from the investigation into it.

Investigation

Police moved quickly. On October 6, 1993, just ten days after the killing, Boston detectives arrested 19-year-old Sean Ellis and his acquaintance Terry Patterson. Prosecutors alleged the two had spotted Mulligan asleep in his vehicle and decided to ambush him to steal his firearm.

The case against Patterson rested largely on latent fingerprint evidence said to have been recovered from the driver's door of Mulligan's Ford Explorer. The case against Ellis was built on witness testimony placing him near the Walgreens that night, together with the alleged recovery of a firearm linked to the killing.

What the defense did not learn until years later was that several of the Boston Police detectives who built the case against Ellis were themselves under federal investigation for serious corruption — including theft, evidence tampering, and ties to drug dealers. Defense lawyers have argued that Commissioners William Bratton and Paul Evans were aware of mitigating information about the investigators' misconduct that was never disclosed to Ellis's defense at trial.

Trial and Verdict

Sean Ellis was tried three times. The first two trials ended in hung juries; in his third trial in 1995, he was convicted of first-degree murder, armed robbery, and two counts of unlawful possession of a firearm, and sentenced to life in prison without the possibility of parole.

Terry Patterson was convicted in February 1995 in a separate trial. His case began to unravel when the Massachusetts Supreme Judicial Court ruled in 2005 that the latent fingerprint identification method used against him was not sufficiently reliable to be admitted as evidence. Patterson's convictions were overturned, and he was eventually freed in 2006 after pleading guilty to reduced charges in exchange for time already served.

Ellis's path took longer. In 2015, a Superior Court judge vacated his murder conviction, citing the prosecution's failure to disclose exculpatory information about the corrupt detectives who had built the case. In December 2018, after 25 years, the Suffolk County District Attorney's Office dismissed the murder and armed robbery charges entirely. His remaining weapons convictions were vacated in 2021.

Today

The killing of Detective John J. Mulligan remains, in the strictest legal sense, unsolved. No one currently stands convicted of his murder, and the Suffolk County District Attorney's Office has not announced any new suspects. For the Mulligan family and for many in the Boston Police Department, that outcome is a continuing source of pain.

For Sean Ellis, the case became a symbol of wrongful prosecution. His story was the subject of the 2020 Netflix documentary series Trial 4, which examined the corruption allegations against the original investigators and the long fight to overturn his convictions. The case continues to be cited in debates over police accountability, prosecutorial disclosure obligations, and the reliability of fingerprint identification in Massachusetts courts.