In the early morning of Sunday, September 15, 1963, four members of the United Klans of America: Thomas Edwin Blanton, Jr.; Herman Frank Cash; Robert Edward Chambliss; and Bobby Frank Cherry, planted a minimum of 15 sticks of dynamite with a time delay under the steps of the church, close to the basement.
At approximately 10:22 a.m., an anonymous man phoned the 16th Street Baptist Church. The call was answered by the acting Sunday School secretary: a 14-year-old girl named Carolyn Maull. To Maull, the anonymous caller simply said the words, "Three minutes", before terminating the call. Less than one minute later, the bomb exploded as five children were present within the basement assembly, changing into their choir robes in preparation for a sermon entitled "A Love That Forgives". According to one survivor, the explosion shook the entire building and propelled the girls' bodies through the air "like rag dolls".
The explosion blew a hole measuring seven feet in diameter in the church's rear wall, and a crater five feet wide and two feet deep in the ladies' basement lounge, destroying the rear steps to the church and blowing one passing motorist out of his car. Several other cars parked near the site of the blast were destroyed, and windows of properties located more than two blocks from the church were also damaged. All but one of the church's stained-glass windows were destroyed in the explosion. The sole stained-glass window largely undamaged in the explosion depicted Christ leading a group of young children.
Hundreds of individuals, some of them lightly wounded, converged on the church to search the debris for survivors as police erected barricades around the church and several outraged men scuffled with police. An estimated 2,000 black people, many of them hysterical, converged on the scene in the hours following the explosion as the church's pastor, the Reverend John Cross, Jr., attempted to placate the crowd by loudly reciting the 23rd Psalm through a bullhorn. One individual who converged on the scene to help search for survivors, Charles Vann, later recollected that he had observed a solitary white man whom he recognized as Robert Edward Chambliss (a known member of the Ku Klux Klan) standing alone and motionless at a barricade. According to Vann's later testimony, Chambliss was standing "looking down toward the church, like a firebug watching his fire".
Four girls, Addie Mae Collins (age 14), Carol Denise McNair (age 11), Carole Robertson (age 14), and Cynthia Wesley (age 14), were killed in the attack. The explosion was so intense that one of the girls' bodies was decapitated and so badly mutilated in the explosion that her body could only be identified through her clothing and a ring, whereas another victim had been killed by a piece of mortar embedded in her skull. All four girls were pronounced dead on arrival at the Hillman Emergency Clinic. The then-pastor of the church, the Reverend John Cross, would recollect in 2001 that the girls' bodies were subsequently found "stacked on top of each other, clung together".
More than 20 additional people were injured in the explosion, one of whom was Addie Mae's younger sister, 12-year-old Sarah Collins,who had 21 pieces of glass embedded in her face and was blinded in one eye.In her later recollections of the bombing, Collins would recall that in the moments immediately before the explosion, she had observed her sister, Addie, tying her dress sash. Another sister of Addie Mae Collins, 16-year-old Junie Collins, would later recall that shortly before the explosion, she had been sitting in the basement of the church reading the Bible and had observed Addie Mae Collins tying the dress sash of Carol Denise McNair before she had herself returned upstairs to the ground floor of the church.