DC’s 87th homicide of the year—25-year-old Antonio Headspeth, shot and killed in an alley near Barry Farm on the east bank of the Anacostia River on October 17—was easily ignored in a busy capital city.

But across town in her Cleveland Park apartment, Laura Amico got to work. Headspeth was shot at 1:30 am. By 9 that morning, Amico had put up a page on her Web site, Homicide Watch D.C., about his death. It included news about the shooting and a map showing where he’d died.

When Headspeth’s mother, Mervia, heard that her son had been killed, she went onto Homicide Watch and wrote in the comments: “WHO KILLLLLLLED MY SON !!!!!!!!SPEeak UPPPPPPPP!!!!!”

Over the next few days, Amico would add Antonio’s obituary, his photo, the police department’s poster announcing a $25,000 reward for information, the names and phone numbers of the two detectives working the case—and more comments.

From Headspeth’s sister, Kathleen McCoy: “Antonio was my brother he had other siblings and a mother and father that love him dearly. Who ever took my brother’s life please turn yourself in.”

And this: “We love you Dad, from Tonio and Renzo.”

Amico’s motto appears at the top of the Homicide Watch homepage: “Mark every death. Remember every victim. Follow every case.”

Below the motto are faces of the fallen. Sweet Elaine Coleman, with fluffy earmuffs and striped scarf; Silvestre Antonio Perez-Agustine, in his snow-dusted parka; Viola Drath in her pearl necklace and silver broach. Lucki Pannell bursting with life; Karen Jordan grinning above her Redskins sweatshirt.

A map pinpoints where each was killed and shows what we know too well: All but a few of DC’s murders take place east of Rock Creek Park.

Amico’s is a solitary mission—to fill a void in how we understand and acknowledge murders in the District. Her site has become a place where police and prosecutors, friends and families of victims, and people who are simply curi...