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The U.S. Supreme Court Monday cleared the way for a new trial for a Georgia man convicted of murder and sentenced to death by an all-white jury, finding that prosecutors intentionally kept blacks off the jury.

"Prosecutors were motivated in substantial part by race," wrote Chief Justice John Roberts, who was joined in the opinion by all but one of the justices. In his dissent, Clarence Thomas, the court's only African-American member, said the court did not have jurisdiction to take up the case.

A view of the U.S. Supreme Court building is seen in Washington, in this October 13, 2015 file photo. JONATHAN ERNST / Reuters

The jury was chosen for the death penalty trial of Timothy Tyrone Foster, who was 18 when he was charged with sexually molesting and killing a 79-year-old widow in Rome, Georgia in 1986. She was white.

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During jury selection, prosecutors used a list of potential jurors that highlighted the names of blacks in green. Five black panelists qualified to serve were the first five on a prosecution list of "definite NO's." And prospective black jurors were noted as "B#1, B#2, and B#3."

Related: U.S. Supreme Court to Consider Racial Bias in Selecting Jurors

Documents obtained after the trial showed that a prosecution investigators said that "if it comes down to having to pick one of the black jurors," one woman specifically named "might be OK."

Prosecutors later said they excluded her because she was too close in age to the defendant. But she was 34. The defendant was 18.

"The focus on race in the prosecution's file plainly demonstrates a concerted effort to keep black prospective jurors off the jury," Roberts said in explaining today's ruling.

Half a century ago, proving bias in jury selection required a defendant to show that prosecutors had systematically excluded blacks over a long period of time. But in 1986, the Supreme Court ruled that each trial was to be examined on its own, a decision intended to further discourage bias in jury selection.

"There's real concern that many black inmates on death row are there because of race. Today's decision shows the court is willing to step in to correct egregious injustices even from decades ago," said Tom Goldstein, a Supreme Court expert and publisher of the SCOTUSBlog website.

Civil rights lawyers had hoped the Supreme Court would use this case to impose further limits on when prosecutors can exclude jurors without saying why. But the ruling did not go that far.

Monday's decision allows Foster to seek a new trial — 30 years after the murder.