Here's more on Ella Ellison:

Ella Mae Ellison - convicted: 1974
In 1973, Boston Police Detective John Schroeder was murdered in the commission of a robbery. The three youths who committed the crime were apprehended within a day. In the course of plea-bargaining, two of the defendants agreed to testify against an accomplice named "Sue." Ella Mae Ellison was eventually arrested as being "Sue." While she knew the defendants, the only evidence against her was their testimony. As part of their agreement with prosecutors, the two plead guilty to second-degree murder. Ellison was convicted of first-degree murder and armed robbery and sentenced to two life terms. In 1976, the two defendants who had testified against Ellison admitted that there had been no fourth participant in the crime; they had invented "Sue." The Supreme Judicial Court subsequently reversed Ellison's conviction and all charges were dropped. Released: 1978

Source: nodp.org (No Death Penalty)

The Boston Globe
March 26, 1980
By Globe Staff

Terrell Walker, 24, formerly of Dorchester, was sentenced to 18 to 20
years in prison yesterday after he pleaded guilty to manslaughter in the slaying of a Boston detective during a Roxbury holdup on Nov. 30, 1973.

Judge Andrew R. Linscott in Suffolk Superior Court imposed the term on Walker after the defendant admitted killing Detective John D. Schroeder during a robbery of the Suffolk Loan Co. on Washington Street.

Walker also was given four concurrent 15 to 20 year terms for four armed robbery indictments. He will be eligible for parole in six years, having already served six years.

Walker was convicted on a first degree murder charge by an Essex Superior Court jury in August 1974. After an appeal process, his conviction was reversed and remanded for a new trial by the US Court of Appeals.

The manslaughter plea was negotiated between Suffolk Dist. Atty. Newman Flanagan and Walker's lawyer, Norman Salkind.

Schroeder was in the Suffolk Loan office when three young men, all armed with handguns, burst into the pawnshop and began scooping jewelry from the display cases.

Detective Sgt. Robert Hudson told Linscott that Schroeder got into a struggle with Walker and was fatally wounded. "He (Schroeder) fell to the floor and the defendant went through his clothes, removed his badge, his wallet and his service revolver."

Besides Walker, Anthony J. Irving, 18, Nathaniel Williams, 21, and Ella Mae Ellison, 27, were arrested and charged with being involved in the robbery. All lived in the Columbia Point Housing Project.

Irving and Williams pleaded guilty to second degree murder and armed robbery charges and testified against both Ellison and Walker in their trials. Irving and Williams are serving life sentences.

Ellison's first degree murder conviction was later reversed by the Massachusetts Supreme Judicial Court and the indictments against her were dismissed when Irving and Williams recanted their testimony naming her as the driver of the escape car.

Due to the recantation of Irving's and Williams' testimony and the deaths of two other prosecution witnesses, Flanagan said he "regrettably had to enter into a plea bargain with defendant's counsel."

Flanagan added that "a jury of (Walker's) peers found him guilty and the Supreme Judicial Court upheld the conviction as did a judge of the US District Court.

"Unfortunately, a circuit court of appeals made new law to the benefit of the defendant and to the detriment of the public, a law prosecutors must live with but with which I do not agree," Flanagan said.

"Nevertheless, I am satisfied the ends of justice, as far as the public is concerned, were met, taking all of the circumstances under consideration."

Schroeder's brother, Walter, a Boston Police patrolman, was shot and killed in a holdup of a Brighton bank on Sept. 23, 1970.