23-year-old Boston University student from China; 8-year-old spectator Martin Richard; and 29-year-old spectator Krystle Campbell. But it also injured more than 170 people: runners, couples, spectators, children. Some are in grave condition; some lost limbs or senses; all their lives will be forever touched by the bombings. Here are some of their stories.
CELESTE AND SYDNEY CORCORAN: HOLDING HANDS
Celeste Corcoran is a hairstylist on upscale Newbury Street, not far from the site of the bombing. Her daughter, Sydney, 17, is a senior at Lowell High School.
Celeste lost both legs below the knee and Sydney has severe injuries as well, cousin Alyssa Carter said in a fundraising page set up at GoFundMe.com. The page had raised more than $320,000 by Thursday evening.
Family members said Celeste was being incredibly positive. Her sister, Carmen Acabbo, told WJAR-TV that Celeste joked she "would be a hairstylist on wheels now."
Sydney has been through tough times before. She was hit by a car while crossing a street less than two years ago and fought to recover from a fractured skull, the Lowell Sun reported.
Nurses at Boston Medical Center were ultimately able to get the mother and daughter into the same hospital room.
"Their beds are pushed together," Carter wrote, "so they can hold hands."
ERIKA BRANNOCK AND NICOLE GROSS: A MOTHER'S GUILT
Carol Downing asks herself if she could have done something differently that would have spared her daughters from Monday's bombing.
Downing was about to cross the finish line in her first Boston Marathon when the bombs went off.
Daughter Nicole Gross, 31, is a personal trainer and triathlete in North Carolina at the Charlotte Athletic Club, where her husband, Michael, is general manager. She helped her mother prepare for the race.
Downing's younger daughter, Erika Brannock, 29, teaches 2-year-olds at a preschool at Trinity Episcopal Church in Towson, Md.
Erika's leg had to be amputated. Nicole's left leg was broken and right ankle was fractured, according to WCNC-TV. Michael had injuries including third-degree burns.
Downing told WCNC that it was supposed to be a fun weekend and that her daughters were so proud of her. Then the "what ifs?" came pouring out.
"What if I hadn't qualified? What if I hadn't come? What if I had gotten sick and couldn't come?" she asked. "What if I had run faster? What if they had stood someplace different?"
AARON HERN: SPECIAL VISITOR
Eleven-year-old Aaron Hern of Martinez, Calif., got a special visitor on Thursday.
The Kiwanis Club in his hometown posted a photo online showing Aaron and his family in his room at Boston Children's Hospital with first lady Michelle Obama. She and President Barack Obama came to Boston for an interfaith service and to visit people injured in the Boston Marathon bombings.
Aaron suffered leg wounds when the bomb went off. He was there with his father, Alan, and younger sister, Abby, to cheer his mother, Katherine, in her first Boston Marathon.
Aaron remained in critical condition Thursday, but The San Francisco Chronicle reported Aaron's mother said in a Facebook message to family and friends that he may be able to leave intensive care as soon as Friday. She said he had successful surgery Wednesday to close his wounds without the need for skin grafts.
MICHELLE L'HEUREUX: LOVES LIFE
Michelle L'Heureux was standing near the finish line to watch her longtime boyfriend finish his 11th Boston Marathon when the bombs exploded, according to her uncle Zoo Cain.
Shrapnel tore into L'Heureux's left shoulder and one of her legs. Falling glass from an adjacent building cut her face. She underwent a second surgery in Brigham and Women's Faulkner Hospital on Wednesday. Cain, who visited his niece in the hospital this week, said metal remains in her leg, but she's expected to recover physically.
The emotional damage could be long-term.
"Her life is altered forever," Cain said, noting his niece described seeing people with missing limbs, blood soaked streets and people running and screaming from what felt like a war zone.
L'Heureux, who Cain said was in her 30s, lives in suburban Boston and works in the finance industry. She is a native of Auburn, Maine.
"Michelle is a very beloved aunt" to her younger sister's daughters and loves her dogs, Cain said.
L'Heureux's passion is horses; Cain said she used to have her own horse and participated in horse shows before moving to the Boston area. He described her as a cheerful person.
"She loves life," he said.
JARROD CLOWERY: 'GET INTO THE STREET'
Jarrod Clowery and his friends were cheering on spectators when he heard the first explosion.
"I got this feeling that we need to get into the street," Clowery said.
Clowery, 35, a carpenter, hopped over one of the metal barricades that separates spectators on the sidewalk from runners on the course when the second blast went off behind him.
"Because I was elevated on the railing, I think I avoided major, major injury," Clowery said, adding that his friends were injured much more severely.
Clowery said his hearing was diminished by about 85 percent. He has shrapnel embedded in the back of his leg and suffered flash burns.
"The Lord was watching over me, somebody was watching over me," Clowery said. "And I feel very blessed."
THE WHITES: FRIENDS BAND TOGETHER
Kevin White says the toughest part of being injured in the Boston Marathon bombing was not being able to find his parents.
White, 35, who lives in Boston and Chicago, suffered shrapnel injuries. His mother, Mary Jo, broke several bones, and his father, Bill, had his right foot amputated. They had just left a restaurant when the bomb exploded about 10 feet away.
White, who was released from Boston Medical Center on Wednesday, says he's looking forward to reuniting with his father, who is in the intensive care unit at another hospital.
Some close family friends have an online fundraising drive to help the White family pay some of the hefty medical bills they are expected to confront during the months.
The initiative had generated more than $18,400 by late Wednesday, reaching in two days nearly its original goal of raising $20,000 in a month.
White said his family is very grateful but urged well-wishers to also donate to The One Fund Boston, the charity established to help all families affected by the bombings.