The US Supreme Court has quashed the murder conviction of a black man on death row in Mississippi because of a prosecutor’s “relentless” efforts to stop African Americans appearing on the jury at successive trials.

Curtis Flowers, 49, has already been tried six times and now could face a seventh trial following the decision by the country’s highest federal court. He has been in jail more than 22 years, ever since his arrest after four people were found shot dead in a furniture store in Winona, Mississippi, in July 1996.

The removal of black jurors deprived inmate Mr Flowers of a fair trial, the Supreme Court ruled in a 7-2 decision written by Justice Brett Kavanaugh. A series of trials stretching back more than 20 years shows Doug Evans, the district attorney, made a “relentless, determined effort to rid the jury of black individuals”, with the goal of an all-white jury in Mr Flowers’ case, Justice Kavanaugh wrote.

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“The numbers speak loudly,” Justice Kavanaugh said in a summary of his opinion that he read in the courtroom, noting that Mr Evans had removed 41 of the 42 prospective black jurors over the six trials. “We cannot ignore that history.”

Mr Flowers was found guilty in his first three trials, but the three convictions were overturned by the Mississippi supreme court due to “prosecutorial misconduct”. The fourth and fifth trials ended in mistrials.

He was convicted again on the sixth trial in 2010, when the jury was made up of 11 whites and one African American and Mr Evans struck off five black prospective jurors – a conviction overturned on Friday.

State prosecutor Doug Evans, centre, at Curtis Flowers’ 2010 trial (The Commonwealth via AP) (Taylor Kuykendall / The Commonwealth via AP)

In a dissenting view, Justice Clarence Thomas called Justice Kavanaugh’s opinion “manifestly incorrect” and wrote that Mr Flowers “presented no evidence whatsoever of purposeful race discrimination”.

Justice Thomas, the only African American currently on the Supreme Court, said: “The state is perfectly free to convict Curtis Flowers again.”

Mr Evans said he remained confident of Mr Flowers guilt but had not yet decided whether the state of Mississippi would order a retrial, according to American Public Media. He denied trying to exclude African Americans from the jury. Mr Flowers’ defence lawyers have argued that witness statements and physical evidence against him are too weak to convict him of the killings of four furniture store workers.

“A seventh trial would be unprecedented, and completely unwarranted given both the flimsiness of the evidence against him and the long trail of misconduct that has kept him wrongfully incarcerated all these years,” said Sheri Lynn Johnson, who represented Mr Flowers at the Supreme Court.

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“We hope that the state of Mississippi will finally disavow Doug Evans’ misconduct, decline to pursue yet another trial and set Mr Flowers free.”

In the course of selecting a jury, lawyers can excuse a juror merely because of a suspicion that someone would vote against their client using “peremptory strikes”, but they have been the focus of complaints about discrimination.

The Supreme Court tried to stamp out discrimination in the composition of juries in the Batson v Kentucky decision in 1986, ruling that jurors couldn’t be excused from service because of their race.