In past years, Krystle Campbell had often gone to cheer runners as they completed the Boston Marathon and this spring she decided to attend the event with a friend, Karen Rand.
When two bombs detonated Monday near the finish line, both women were knocked unconscious.
Ms. Campbell died and her family was mistakenly told she was only wounded.
Her father was taken to a hospital bed, expecting to see his injured daughter. Instead, it was Ms. Rand, still sedated from surgery.
"My son was devastated," Ms. Campbell's grandmother, Lillian, told NBC.
Ms. Campbell was one of three people who died in the terrorist attacks, all three young people whose lives were cut short as they came to cheer the marathoners.
The other two victims were Martin Richard, 8, whose mother and sister were also badly injured.
A heart-tugging past photo of Martin holding a peace sign had gone viral, a symbol of the senselessness of the tragedy.
The third victim, Lu Lingzi, a Chinese citizen in her early 20s, was a graduate student at Boston University.
According to her LinkedIn profile, she grew up in Shenyang in northeast China, had worked as a financial analyst and was enrolled in the master's program in mathematics and statistics.
On Monday, all three were in the crowd along Boylston Street, off Copley Square, near the finish line.
Ms. Campbell, a 29-year-old steakhouse manager, came with her friend to take a picture of Ms. Rand's boyfriend, who was taking part in the race.
Then the bombs went off and for hours there was no news of the two women.
"Worrying like crazy about … Karen, who was in the area where one of the bombs went off at the marathon. Phone lines are down and we can't reach her," Ms. Rand's sister-in-law, Cheryl Rand Engelhardt, wrote on Facebook.
The detonation had broken Ms. Rand's left leg and severed an artery. Her eardrums were also broken. She underwent surgery at Boston's Massachusetts General Hospital.
According to her sister-in-law, Ms. Rand was carrying Ms. Campbell's identification papers so that when she came out of surgery, the hospital thought she was her friend.
Ms. Campbell's father, William, had initially been told that his daughter might lose a leg.
He was stunned when instead of his daughter he saw Ms. Rand in the hospital bed, her grandmother Lillian told NBC, adding that officials would only show him a photo of his daughter to confirm the young woman's death.
"You couldn't ask for a better daughter," Ms. Campbell's tearful mother, Patricia, told reporters outside the family home. "I can't believe this has happened. She was such a hard worker at everything she did."
She paused then said: "This doesn't make any sense."
Meanwhile, in the Dorchester suburb of Boston, people were grieving the death of third-grader Martin Richard.
His mother, Denise O'Brien Richard, who reportedly suffered a brain injury, and his six-year-old sister Jane, who had a serious leg injury.
"My dear son Martin has died from injuries sustained in the attack on Boston. My wife and daughter are both recovering from serious injuries," Martin's father, Bill, said in a statement released to the media.
"We thank our family and friends, those we know and those we have never met, for their thoughts and prayers. I ask that you continue to pray for my family as we remember Martin," he added.
A year ago, following a class where they learned about Mahatma Gandhi, Martin's teacher had taking the class to Boston city hall for a peace march.
A photo from that occasion shows Martin holding up a blue cardboard sign that he made. Next to two hearts, it said: "No more hurting people. Peace."