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'Trial 4': Who was John Mulligan? Slain Boston Police detective may have been involved in crime and corruption

A few years after Mulligan's murder, his colleagues were accused of being corrupt cops who falsified search warrants to gain entry to apartments of known drug dealers

Netflix's new limited docuseries, 'Trial 4', is about a man who says he was wrongly convicted of murdering a police officer when he was just 19 years old. The eight-part docuseries will feature Sean Ellis as he faces his fourth trial — 25 years after he was first tried in 1995 — as he may end up going back to prison. Ellis had been previously sentenced to life and was released when new evidence came to light.

The docuseries tells the story of Sean K Ellis, who has spent 22 years in prison after being convicted in the 1993 murder of Boston police detective John Mulligan. Ellis was put on trial three times within a year, and now faces his fourth trial – which could see him back in prison for life. After his first three trials for armed robbery and first-degree murder, Ellis, aged 19 at the time, was found guilty in 1995 and sentenced to life.

His first two trials resulted in a hung jury. In 2015, Ellis was freed from jail on bail, after a judge ordered a new trial saying evidence about how the case had been handled had been withheld from the defense. Ellis, who has always maintained his innocence, is now awaiting trial number four and the prospect of life behind bars again.

Mulligan had been a Boston police detective for 17 years before his death. In the early morning of September 26, 1993, he was working a paid security detail at a Walgreens pharmacy in the Roslindale section of Boston, a detail that he worked several times a week. 

However, news reports from The Boston Globe's award-winning Spotlight team suggested that the Boston Police Department had many problems at the time. According to the docuseries, a 1991 report stated that the Boston police had serious deficiencies in solving crimes — there was inattention to detail, officers would fail to secure the scene of the crime, a lack of forensic evidence collection and heavy reliance on eyewitnesses who could be unreliable.

A few years after Mulligan's murder, his colleagues were accused of being corrupt cops who falsified search warrants to gain entry to apartments of known drug dealers and undocumented immigrants. According to the report, "Once in, they demanded money and drugs and, in exchange, either did not arrest the dealers or got their charges reduced. By not showing up for their scheduled court appearances, the officers extorted additional payments when the charges were dismissed." The three officers were Kenneth Acerra, Walter Robinson and John Brazil.

Further investigation revealed that the corrupt scheme had netted the corrupt cops hundreds of thousands of dollars and had gone on for over a decade — before, during and after the Mulligan murder and trials. In March 1997, Assistant US Attorney Ted Merritt indicted Acerra and Robinson on over 40 counts of perjury and armed robbery while Brazil was granted immunity from prosecution in exchange for informing on his colleagues.

Acerra and Robinson at first vigorously denied all charges, but a year later pleaded guilty in return for reduced sentences. It is believed that Mulligan may have been part of Acerra's and Robinson's criminal scheme. A few weeks before he was murdered, the slain detective reportedly committed an armed robbery of Boston drug dealer Robert Martin's two Commonwealth Avenue apartments with Acerra and Robinson.

'Trial 4' is now streaming on Netflix.