Birmingham became the center of the civil rights movement in spring 1963, when Martin Luther King Jr. and his supporters in the Southern Christian Leadership Conference arrived with a plan they called “Project C”—for confrontation. At that point, Black Americans were forced to attend different schools, eat in different restaurants, live in different neighborhoods and drink from different water fountains than their fellow white citizens.

The city of about 340,000, which King called the most segregated in America, had even abandoned its minor league baseball team rather than watch it play against integrated competition. Meanwhile, around 50 bombings and dozens of cross-burnings had occurred there since 1947, earning Birmingham the nickname “Bombingham.”

King kicked off Project C with sit-ins at segregated downtown lunch counters and marches on City Hall. After being arrested on April 12 for violating a court injunction that prohibited such demonstrations, King penned his famous “Letter from Birmingham Jail,” in which he lambasted those “more devoted to ‘order’ than to justice.” Children’s protests in early May, which included more than 1,000 children—some reportedly as young as eight years old—were then brutally suppressed with police dogs, nightsticks and high-powered fire hoses, images of which appeared in newspapers and on television sets around the country.

Under pressure from the administration of President John F. Kennedy, local business leaders soon agreed to desegregate on a staggered timetable. Kennedy also sent a civil rights bill to the U.S. Congress that, when it passed in 1964, banned segregation in public places and employment discrimination on the basis of race.

“Now the time has come for this nation to fulfill its promise,” he said in a June 1963 televised address. “The events in Birmingham and elsewhere have so increased the cries for equality that no city or state or legislative body can prudently choose to ignore them.”

By late summer 1963, Birmingham’s golf courses, restaurants, department stores and schools had begun integrating. Nonetheless, race-related bombings continued, including one at the home of King’s brother that sparked a riot. On September 8, bomb threats were called in to the 16th Street Baptist Church, which had served as a staging ground for the spring demonstrations.

Exactly a week later, at 10:22 a.m., an estimated 10 to 15 sticks of dynamite exploded underneath the church’s stairs, knocking a gaping hole in a restroom wall, creating a crater over two-feet deep in the basement and spraying debris all over the building. The blast was so powerful, in fact, that it blew a motorist from his car, destroyed vehicles parked outside and shattered windows blocks away.

None of the previous Birmingham bombings going back to 1947 had actually killed anyone. But this time around, four girls—14-year-olds Addie Mae Collins, Cynthia Wesley and Carole Robertson and 11-year-old Denise McNair—were found dead in a basement bathroom, where they had gone prior to an 11 a.m. “Youth Day” service. One of the four was decapitated and all reportedly had their clothes blown off. A fifth girl in the bathroom, Sarah Collins, 12, was permanently blinded in one eye, and more than 20 people elsewhere in the church also suffered injuries.

That afternoon, as the authorities and projectile-throwing protestors clashed outside of the church, two more deaths occurred. Johnnie Robinson, 16, was allegedly hurling rocks at a white segregationist’s car when police ordered him to stop. Robinson instead took off running, and a cop, who was never indicted, shot him in the back. Another boy, Virgil Ware, 13, was riding on the handlebars of his brother’s bicycle when a 16-year-old white Eagle Scout shot him in the face and chest. The Eagle Scout, who had just attended a segregationist rally, received only probation for the crime.

Robinson and Ware would be largely forgotten to history, but not the young girls. At a funeral for three of them—the family of the fourth preferred a small, private service—King praised them as “martyred heroines of a holy crusade for freedom and human dignity.” The civil rights leader also used the occasion to criticize “every minister of the gospel who has remained silent behind the safe security of stained-glass windows” and “every politician who has fed his constituents the stale bread of hatred and the spoiled meat of racism.” Alabama’s pro-segregation governor, George Wallace, became a particular target for criticism. “The blood of our little children is on your hands,” King told him in a telegram.

Alas, neither King’s oratory nor the passage of the Civil Rights Act of 1964 prevented the wheels of justice from moving slowly. The FBI’s Birmingham field office, which had launched an investigation into the bombing, recommended that at least four suspects be prosecuted. But FBI Director J. Edgar Hoover, a well-known opponent of the civil rights movement, blocked such action from taking place. State authorities likewise refused to move forward, except to charge KKK member Robert Chambliss and two others with possession of dynamite.

The case reopened in 1971 following the election of Alabama Attorney General Bill Baxley, and six years later a jury convicted Chambliss of murder, in part due to testimony that his niece gave against him. He died in prison in 1985. Two more suspects, Thomas E. Blanton Jr. and Bobby Frank Cherry, were convicted in 2001 and 2002, respectively—both received life sentences—whereas the fourth suspect, Herman Frank Cash, died in 1994 without ever facing charges.