It was a double celebration: Christmas, and the Moores’ 25th anniversary. Harry T. and Harriette Moore celebrated the way they had 25 years before, cutting the cake together like newlyweds. They had no idea that the tender moment would be among their last. As they settled in to their bed to sleep that evening in 1951, a massive explosion tore through their bedroom.

Within hours, Harry T. Moore was dead. Within days, his wife was, too. With the death of the Moores, the nascent Civil Rights Movement got its first martyrs.

The Moores had been murdered, victims of an improvised explosive device made with dynamite and shoved beneath their bedroom floor. It seemed like a simple case: Harry T. Moore had been fighting segregation and racism in the Jim Crow South for years, making plenty of enemies along the way.

But though the Moores’ murders produced an immediate list of suspects, even a deathbed confession, no arrest was ever made in their case. To this day, it remains unsolved, despite convincing evidence that the Moores were killed by members of the Ku Klux Klan.

The Moores had already sacrificed for their beliefs. Born in segregated Florida, both had been fired from their jobs as teachers after they founded a local chapter of the NAACP. At the time, segregation was the norm in former Confederate states, and the Moores and other black Southerners had few opportunities for economic or social advancement. Harry had met his wife after getting a job at a school for black children in Cocoa, Florida. The Titusville “Negro School” offered black students a chance at an equal education, and Moore eventually became its principal.

But his political activism—including founding the Brevard County NAACP and filing a lawsuit to try to force Florida to pay black teachers as much as white ones—put him in the crosshairs of Florida’s racist political establishment and locals. He was fired from his job as principal, and Harriette was fired from her teaching position. In response, Harry became an organizer for the NAACP, traveling Florida, registering black voters, and investigating lynchings across the state.

As Harry’s activism became more and more visible, he became a target. Lynching was still alive and well in Florida, and vigilantes often took the law into their own hands, even in the few cases where the legal system ruled in favor of black people.

One such case was the 1949 Groveland rape case, in which four young black men were accused of raping a white woman. Three of the suspects were brutally beaten by police after being taken into custody; a fourth, Ernest Thomas, was shot over 400 times by a posse that chased him down after he escaped custody. The surviving men all contested the accusations and were all convicted by all-white juries.

Harry organized a campaign to support their appeals, and two of the cases reached the U.S. Supreme Court. Thurgood Marshall represented the defendants and they won the case. But they were subsequently shot by Sheriff Willis McCall, who transported the men from prison to the location of their new trial. One defendant, Samuel Shepherd, died. The other, Walter Irvin, was gravely wounded, but survived. He accused McCall of murder. Harry Moore campaigned hard against McCall, calling for him to be suspended and indicted.

Harry’s murder took place just six weeks after he called for McCall’s indictment. As the outraged black community mourned the deaths and called for justice, the FBI began investigating the murders. They conducted hundreds of interviews, even building a scale model of the Moores’ home and blowing it up with dynamite. Immediately, the FBI homed in on the Ku Klux Klan, which was large and active in the area. Many of the area’s most important business, political and law enforcement figures were members, and the FBI determined that they were aware of Harry and his activities.

They identified two suspects, both members of the Klan: Tillman H. Belvin and Earl J. Brooklyn. An informant told the FBI that Brooklyn had shown them a floorplan of the Moore home, and another witness told the FBI that they had seen both men asking about the Moores’ address. Another suspect, Joseph Neville Cox, killed himself after two interviews with the FBI.

The case looked like it was close to being solved—but then it fell apart. Though the suspects and their Klan activities were well known, suspected witnesses refused to talk. Eyewitnesses suddenly refused to testify or backpedaled their claims. Then, Belvin and Brooklyn died, both of natural causes.

It seemed like the true story behind the murders would never be told. Then, in 1978, the Brevard County sheriff reopened the investigation. A local citizen who spoke out loudly against the renewed investigation, Edward L. Spivey, was dying of cancer. He gave extensive testimony and ended up revealing that Cox had been responsible for planting the dynamite. Though officials became convinced that Spivey had been at the house when Cox planted the bomb, they never prosecuted because the sheriff who had reopened the case, Roland Zimmerman, lost his re-election bid. Spivey died soon after. He was never prosecuted.

The case has been reopened multiple times since the 1970s. In 1991, Florida’s governor ordered a new investigation. Though Klan informants cooperated with the investigation, it did not result in any indictments. In 2004, the Florida Attorney General ordered another investigation. But though they interviewed over 100 people, they came to the same conclusion as others: that the four previously suspected men had carried out the bombing. All were dead. In 2008, the FBI came to the same conclusion. In 2011, an attempt to reopen the case yet again failed.

Today, the Christmas Day bombing is thought of as the first murder in the Civil Rights Movement. Years before figures like Martin Luther King, Jr. or Medgar Evers were killed, the Moores paid the price for their activism. The investigation of their deaths followed a pattern that had been set for centuries: Instead of helping ferret out the murderers, white Southerners rallied around the perpetrators. Few came forward to give information that could help build a case, even when the perpetrators’ identities were known.

The reforms the Moores fought so hard to achieve would have helped people like them find some hope of a fair trial in a just legal system. But they died before the Civil Rights Movement bore fruit. In the Moores’ case—and those of thousands of black people who were lynched or murdered  with no legal resolution—justice would never be served.