In many cases, the exonerations call into question the veracity and reliability of police officers and prosecutors who have stuck life sentences on scores of Bostonians throughout their careers.

It’s a scenario that was highlighted by the Netflix documentary “Trial 4,” which chronicles Sean K. Ellis’ 22-year battle to be freed after he was convicted of killing Boston Police Detective John Mulligan in a trial marred by witnesses who were coerced, witnesses who were paid for their testimony, exculpatory evidence police hid from defense attorneys, and police investigators who appeared more motivated by a desire to cover for their own crimes than to solve the murder of a colleague.

In one of the more telling moments of the Ellis documentary, filmed just weeks before reform-oriented Suffolk County District Attorney Rachael Rollins took office in January 2019, Boston Police Commissioner William Gross contends that Ellis was guilty of Mulligan’s murder during a December press conference to announce the district attorney’s decision not to re-try Ellis, who had been freed in 2017.

“As you heard the district attorney state, Sean Ellis is culpable,” Gross tells reporters. “What does the evidence look like after 25 years? The witnesses and their strength, and the decision was based upon that, not innocence at all.”

Despite a preponderance of evidence to the contrary, including three accounts of witnesses who informed police that another officer had threatened to kill Mulligan before he was shot dead in 1993, police and prosecutors in 2018 seemed unwilling to entertain the possibility that their colleagues convicted the wrong man.

Two of the officers who produced physical evidence and found witnesses linking Ellis to the crime, Walter Robinson and Kenneth Acerra, were convicted in 1998 of robbing drug dealers. On more than one occasion, Mulligan was alleged to have joined them in the robberies. A third officer critical to the investigation, John Brazil, was granted immunity in exchange for his testimony for the prosecution in Robinson and Acerra’s case.

Another detective who testified against Ellis, Daniel Keeler, in 2005 resigned as head of the Homicide Department not long after he admitted in court that he made false statements in an affidavit and on a police report, and reporters documented a string of cases in which people Keeler put away were later exonerated.

Keeler was once known as “Mr. Homicide” for his record of securing convictions in murder cases. But once an officer has been caught uttering an untruth, it can cast doubt on his or her entire record of convictions.