Friends of those killed say they are devastated by the senseless deaths.
"She was the best," Campbell's distraught mother, Patty, told reporters on Tuesday. "You couldn't ask for a better daughter."
The family is heartbroken and still in shock, Patty Campbell said as she tried to read a statement on the family's porch.
Everyone loved Krystle, she said.
"She had a heart of gold. She was always smiling," Patty Campbell said as her son, Billy, clutched her with his right arm.
Krystle's grandmother said the 29-year-old was a special kind of person who nurtured deep friendships.
"Oh, she was a beautiful girl," Lillian Campbell told CNN's Jake Tapper. "She was very happy, outgoing, a hard worker."
Lillian Campbell said her granddaughter even lived with her for a year and a half and was "great with me."
Her granddaughter was always willing to help someone in need, she said.
"And she was, she was just beautiful. She was a fun-loving girl," Campbell said.
Krystle Campbell once worked at Summer Shack, a seafood restaurant in the Boston area that posted a statement on its Facebook page saying she was beloved.
"She was an incredible woman, always full of energy and hard at work, but never too tired to share her love and a smile with everyone," the post said. "She was an inspiration to all of us. Please keep her and her family in your thoughts and prayers."
According to the Boston Globe, Campbell had taken a job with Jimmy's Steer House in Arlington.
The Globe reported that Campbell often went to the see the marathon runners.
"She's been doing it since she was a little girl," Lillian Campbell told the newspaper. "She didn't miss a marathon, watching it at the finish line."
Campbell was a 2001 graduate of Medford High School, the town's mayor, Michael McGlynn, said.
CNN affiliate WHDH
reported that the Campbells are longtime residents of Medford.
Martin Richard, 8, Dorchester, Massachusetts
Like many young boys in New England, Martin Richard
loved his Boston Red Sox and the Bruins.
"He wore his (Red Sox second baseman) Dustin Pedroia shirt to school last week," neighbor Bill Forry told CNN's Anderson Cooper.
Martin was a terrific athlete, too, Forry said, but he was also a very good student who would help others who were having trouble with homework.
"A quiet kid, but a compassionate kid -- and somebody who was a leader," Forry said.
Martin attended the Neighborhood House Charter School.
He "was a bright, energetic young boy who had big dreams and high hopes for his future," the school said in a statement. "We are heartbroken by this loss."
His father, William Richard, released a statement asking people to "continue to pray for my family as we remember Martin."
A neighbor, Jane Sherman, said that the Richard family is a "typical all-American family" and that Martin and his little brother always loved to play in their yard, no matter the weather.
Richard's mother and sister are recovering from serious injuries sustained in the bombing, the father said.
Lingzi Lu, China
Like thousands of others, the graduate student from China crowded with friends around the finish line at the Boston Marathon to cheer on the runners.
She had moved to the city in time for the fall semester, making friends and soaking up new experiences.
The marathon was a chance to be a part of an annual ritual so cherished by Bostonians. And so she went to Copley Square with two friends in tow.
They were in the wrong place at the wrong time.
The day she died, Lingzi Lu, posted a picture of her breakfast to the Chinese social media website Weibo -- bread and a bowl of fruit.
"My wonderful breakfast," she commented in English with a smiley face emoticon.
Boston University's president announced the graduate student's death in an open letter published on the school's website and confirmed that her friend was wounded.
"Our hearts and thoughts go out to the family and friends of both victims," wrote college President Robert Brown.
Lu had worked hard to achieve. She won an academic scholarship to the Beijing Institute of Technology, where she received accolades for her math skills.
She went on to Boston University to pursue that passion and was working on a master's degree in statistics.
She would have known how slim the chances were that something like this could happen in a crowd of thousands cheering the runners on a sunny day.
Sean Collier, 27, Massachusetts
Even those who barely knew Sean Collier filled a tribute page set up Friday with memories of his easy laugh and ever-present smile, his desire to help others and his motivation to be a good police officer.
"He had a really great smile. I'll always remember that," Kristina Lozoya, a student volunteer with the MIT emergency medical service, told CNN. "He was always laughing. He loved his job."
While his brother, Andrew, became a machinist in the engine department at Hendrick Motorsports, one of the major NASCAR racing teams, Sean followed a different path.
"Sean was one of these guys who really looked at police work as a calling," MIT Police Chief John DiFava said in a statement posted on the university website. "He was born to be a police officer."
Collier died a police officer, shot to death in his squad car on the MIT campus.
In a statement, his family asked for privacy.
"We are heartbroken by the loss of our wonderful and caring son and brother," the statement said. "Our only solace is that Sean died bravely doing what he committed his life to -- serving and protecting others."
President Barack Obama praised Collier following the conclusion of a daylong manhunt for the second of two marathon bombing suspects, saying he was a dedicated officer.
"We are grateful to him," Obama said.
After the blasts near the finish line of the iconic road race, Collier's slaying further traumatized many in the MIT community who had gotten to know him during his 15 months on the job.
"The loss of Officer Collier is deeply painful to the entire MIT community," MIT President L. Rafael Reif said on the university website.
Ben Artin, an EMS volunteer, told how Collier proposed arranging a social event to help broaden the relationship between the campus police and the emergency service workers.
"Sean was rare in the degree to which, while working for MIT PD, he wanted to socially engage with the students," Artin wrote to CNN, adding that "I hope that we follow through on this proposal as we heal."