WASHINGTON (AP) — The U.S. Navy has exonerated 256 Black sailors who were found to be unjustly punished in 1944 following a horrific port explosion that killed hundreds of service members and exposed racist double standards among the then-segregated ranks.

On July 17, 1944, munitions being loaded onto a cargo ship detonated, causing secondary blasts that ignited 5,000 tons (4,535 metric tonnes) of explosives at Port Chicago naval weapons station near San Francisco.

The explosion killed 320 sailors and civilians, nearly 75% of whom were Black, and injured another 400 personnel. Surviving Black sailors had to pick up the human remains and clear the blast site while white officers were granted leave to recuperate.

The pier was a critical ammunition supply site for forces in the Pacific during World War II, and the job of loading those ships was left primarily to Black enlisted sailors overseen by white officers.

Before the explosion, the Black sailors working the dock had expressed concerns about the loading operations. Shortly after the blast, they were ordered to return to loading ships even though no changes had been made to improve their safety.

The sailors refused, saying they needed training on how to more safely handle the bombs before they returned.

What followed affected the rest of their lives, including punishments that kept them from receiving honorable discharges even as the vast majority returned to work at the pier under immense pressure and served throughout the war. Fifty sailors who held fast to their demands for safety and training were tried as a group on charges of conspiracy to commit mutiny and were convicted and sent to prison.

The whole episode was unjust, and none of the sailors received the legal due process they were owed, Navy Secretary Carlos Del Toro said in an interview with The Associated Press.

It was "a horrific situation for those Black sailors that remained," Del Toro said. The Navy's office of general counsel reviewed the military judicial proceedings used to punish the sailors and found "there were so many inconsistencies and so many legal violations that came to the forefront," he said.

Thurgood Marshall, who was then a defense attorney for the National Association for the Advancement of Colored People, defended the 50 sailors who were convicted of mutiny. Marshall went on to become the first Black justice on the Supreme Court.

On Wednesday, the 80th anniversary of the Port Chicago disaster, Del Toro signed paperwork officially clearing the sailors, who are now deceased. Del Toro handed the first pen to Thurgood Marshall Jr., the late justice's son.

The exonerations "are deeply moving," Marshall Jr. said. "They, of course, are all gone, and that's a painful aspect of it. But so many fought for so long for that kind of fairness and recognition."

The events have stung surviving family members for decades, but an earlier effort in the 1990s to pardon the sailors fell short. Two additional sailors were previously cleared — one was found mentally incompetent to stand trial, and one was cleared on insufficient evidence. Wednesday's action goes beyond a pardon and vacates the military judicial proceedings carried out in 1944 against all of the men.

"This decision clears their names and restores their honor and acknowledges the courage that they displayed in the face of immense danger," Del Toro said.

The racism that the Black sailors faced reflected the military's views at the time — ranks were segregated, and the Navy had only reluctantly opened some positions it considered less desirable to Black service members.

The official court of inquiry looking into why the explosion occurred cleared all the white officers and praised them for the "great effort" they had to exert to run the dock. It left open the suggestion that the Black sailors were to blame for the accident.

Del Toro's action converts the discharges to honorable unless there were other circumstances surrounding them. After the Navy upgrades the discharges, surviving family members can work with the Department of Veterans Affairs on past benefits that may be owed, the Navy said.

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