Who would do such a thing? How could anyone so callously bomb innocent people? Why?

Those questions have reverberated for the last two weeks. They reached a peak over the weekend after law officers apprehended one of the brothers who are suspected of placing two pressure cooker-stuffed knapsacks near the Boston Marathon finish line, causing explosions that killed three people, maimed many others, threw the city into chaos and sent ripples of fear across New England and beyond.

The questions are natural as people try to comprehend incomprehensible acts.

But we shouldn’t imagine this is unprecedented. Domestic terrorism is a shameful part of our nation’s history.

On a brilliantly sunny Saturday morning, I stood across from a place where such a thing happened — almost 50 years ago.

The bomb that exploded Sept. 15, 1963, in the basement of the Sixteenth Street Baptist Church in downtown Birmingham, Ala., killed four girls.

The dynamite bundle placed under the ladies’ restroom took the lives of Denise McNair, who was 11, and a trio of 14-year-olds: Addie Mae Collins, Carole Robertson and Cynthia Wesley. Unsuspecting churchgoers weren’t even safe from the Ku Klux Klan while in their church restroom.

Incomprehensible.

Just as unfathomable was the time it took to hold those who committed this heinous crime responsible for their hateful actions.

Robert Chambliss was convicted in 1977, Thomas Blanton Jr. in 2001 and Bobby Frank Cherry in 2002.

Birmingham has changed, but it hasn’t forgotten. Nor should it.

Catty-corner to the church, Kelly Ingram Park peacefully invites visitors, though it holds a wealth of sobering history lessons.

As Boy Scouts from Center Point, Ala., who are working on their history merit badges walked up to a statue of the Rev. Martin Luther King Jr., a voice rang out, “It’s Martin. Hey, Martin.”

It was 50 years ago, on Good Friday, that King, who had come to support local residents’ efforts at racial integration, was arrested for parading without a permit. That April 16, he wrote his “Letter from Birmingham Jail.”

“Injustice anywhere is a threat to justice everywhere,” he wrote.

And this: “When you go forever fighting a degenerating sense of ‘nobodiness’ — then you will understand why we find it difficult to wait.”

That May, public safety Commissioner Bull Connor had his officers arrest thousands of children who marched in the park. They just wanted their rights as Americans. Police dogs and fire hoses were aimed at peaceful protesters.

A stark reminder today: One pathway in the park runs between stone walls with sculptures of barking dogs jutting out as though lunging at passers-by.

Air Force Junior ROTC cadets from West Lowndes High School in Columbus, Miss., were visiting as part of their study of the Civil Rights Act.

Sophomore Bra’Tavious Reed told me she was taken by seeing what “people actually went through so we can have the freedom to do the things that we do today.”

Her fellow student Courtney Gillespie, also a 16-year-old sophomore, said some of her peers don’t really understand the past. “Coming here helped me appreciate it more,” she said.

At the Birmingham Civil Rights Institute, an absorbing museum across from the church and park, an exhibit recounts that the city once was known as “Bombingham” because of the dozens of bombings of homes and churches from the late 1940s into the mid-60s that were clearly intended to discourage black residents from working for equal rights.

Recalling that history doesn’t help much to explain what might have motivated the Boston bombers or how they could have attacked innocents.

But it is a reminder that the human capacity for violence is neither new nor limited to those we might perceive as being “other.”

The human capacity for good is greater, though, and our freedom depends on us continually working to make sure it prevails.