Eight-year-old Martin Richard was a happy and polite child, always smiling and willing to jump in when his parents or teachers sought his help for a community gathering: cleaning trash out of a neighborhood park, decorating a parade float or heading to a peace rally.

At a recent school peace parade, a beaming Martin held up a banner on which he’d written: “No more hurting people.”

On Tuesday night, his family, friends and neighbors gathered at their Dorchester church to grapple with their collective hurt at the third-grader’s death in the bombings in central Boston. He was killed when one of two explosive devices detonated just steps from where his parents had brought him and his siblings to cheer on racers in the Boston Marathon.

“He was so polite, composed, older than his years really,” neighbor and family friend Christina Keefe said of Martin on Tuesday. “I can see him now, holding his mom’s arm as she took them on their walks around the neighborhood. . . . The neighborhood is reeling.”

A few miles away, friends and co-workers of Krystle Campbell, a restaurant catering manager, were holding a similar grief-stricken gathering at the home of her best friend. In a cruel twist of mistaken identity, the 29-year-old’s parents and friends had been told Monday that Campbell was being treated for a serious but survivable leg injury after the blasts. They learned Tuesday that she had died.

A friend, Karen Rand, who accompanied Campbell to watch the marathon, had been carrying Campbell’s photo identification in her pocket at the time of the blast, and hospital staff mistook her for her friend.

Friends and neighbors described Campbell as a hard-working and generous daughter and friend. Yann Kumin, a longtime friend and football coach at a nearby high school, said Campbell’s close-knit group of friends was “just heartbroken,” in part because so many of them had relied on Campbell in times of personal or financial crisis.

“She was the rock of her family, she was the rock for many of her friends,” he said, his voice breaking as he described her. “Krystle loved people. Her hobby was people.”

Campbell, a 2001 graduate of Medford High School, had worked for several years as a manager at the Summer Shack restaurant in Boston’s Back Bay area and managed corporate catering in various locations. A longtime neighbor and friend of Campbell’s parents said their daughter had been a sweet and generous child who grew up to be a giving and lovely daughter.

“She was a good-living kid who did everything to help her parents,” said the neighbor, who asked not to be named.

Boston University officials said Tuesday afternoon that a graduate student there who had been watching the race was the third fatality. The victim’s name was not released.

The Richard family also had a reputation for helping anyone in need. Martin’s father, Bill Richard, the vice president of an environmental testing company, is known for his extensive volunteering to restore main street businesses in nearby Peabody Square, friends said. Martin’s mother, Denise Richard, is a librarian at her children’s neighborhood charter school.

As desperate, curt text messages ricocheted among them Monday evening, friends and neighbors began to hear that Martin was hurt in the blasts. Many gathered at a neighborhood restaurant to learn what they could and offer help. They also heard more bad news: Denise was being operated on for a serious brain injury; her youngest child, Jane, had a traumatic injury to her leg; and Martin had died at the scene.

“Knowing Denise, she probably had some posters for them to hold up” as runners came by, Keefe said. “It’s all just so awful.”

Julie Tate, Alice Crites and J. Freedom du Lac contributed to this report.