Amid the panic and fear, seconds after two bombs exploded at the Boston Marathon, hundreds ran from the chaos while first responders, medics and Good Samaritans ran toward the carnage.
"My first instinct was to just run across the street... and start helping out the people," said Carlos Arredondo.
Arredondo helped rescue Jeff Bauman Jr., a man who lost both of his legs. Bauman Jr. was cheering his girlfriend on in the race. He survived the trauma after people rushed him away from the explosion site in a wheelchair.
Rescuers took the 27-year-old victim to Boston Medical Center, but doctors had to amputate his legs because of extensive vascular and bone damage, a Facebook post from his father said Tuesday.
"Unfortunately my son was just in the wrong place at the wrong time," the elder Jeff Bauman wrote.
The son also had to have more surgery later because of fluid in his abdomen. His condition improved later.
"I just can't explain what's wrong with people today to do this to people," the father wrote. "I'm really starting to lose faith in our country."
Former New England Patriot Joe Andruzzi carried an injured woman to safety.
"I was just looking to help in any which way I can," Andruzzi said.
And doctors -- exhausted from completing the 26-mile race -- kept running back and forth rescuing patients.
"If you want to know who we are... what America is... how we respond to evil... That's it," President Barack Obama said.
"The bomb went off... And of course... immediately came to the hospital," said David King, trauma surgeon, Massachusetts General Hospital, who had just finished running the marathon when he went into surgery. "I walked into the emergency room. I thought to myself, 'This is exactly like I was back in Iraq or Afghanistan."
Many of the victims lost limbs, including two of Liz Norden's sons.
"they both lost a leg... below the knee," she said.
Nicholas Yanni nearly lost his wife in the blast as he rushed to make a tourniquet.
"Some shrapnel had hit her leg. I grabbed shirts off the rack... and started wrapping her leg up," he said.
And doctors, who have been working non-stop in Boston-area hospitals since the explosions - say it's the heroic efforts that have made the biggest impact.
"Because of the rapid response... life was able to be saved," said Dr. George Velmahos, chief of trauma service, Massachusetts General.
Marathoners without medical training also didn't quit. They continued running to hospitals to donate blood.
The twin bombs at the Boston Marathon killed three people and wounded more than 170 on Monday. Here are the stories of those killed and some of the injured.
A LONG WAY FROM HOME
A Boston University graduate student was one of the three people killed in the bombings at the Boston Marathon, the school said Tuesday.
The Shenyang Evening News, a state-run Chinese newspaper, identified her Wednesday as Lu Lingzi.
Phoenix Satellite Television Holdings, a Hong Kong-based broadcaster with ties to the Chinese government, said she was from the northeastern Chinese city of Shenyang and a graduate student in statistics.
In a statement late Tuesday afternoon, the school said it was not releasing the name or any other information about the student. The Chinese Foreign Ministry and Consulate General in New York did not identify the victim at the request of the family.
The Boston University statement says the student was with two friends who were watching the race at the finish line, not far from campus. One of the friends, also a graduate student at the university, was injured and is at Boston Medical Center in stable condition.
A team led by Deputy Consul General Ruiming Zhong was in Boston to investigate and assist relatives of the victims, a statement from the consulate said.
THE RICHARDS: A FAMILY INJURED, IN MOURNING
Neighbors and friends remembered 8-year-old bombing victim Martin Richard as a vivacious boy who loved to run, climb and play sports like soccer, basketball and baseball.
The boy's father, Bill Richard, released a statement thanking family, friends and strangers for their support following his son's death Monday. Richard's wife, Denise, and the couple's 6-year-old daughter, Jane, also suffered significant injuries in the blasts.
The family was watching Monday's race and had gone to get ice cream before returning to the area near the finish line before the blasts.
Denise Richard works as a librarian at the Neighborhood House Charter School, where Martin was a third-grader and Jane attends first grade. Counselors were being made available to staff and students.
"I just can't get a handle on it," family friend Jack Cunningham said of the boy's death. "In an instant, life changes."
KRYSTLE CAMPBELL: CHEERING ON FRIENDS
Krystle Campbell was a 29-year-old restaurant manager from Medford. Her father, 56-year-old William Campbell, described her as "just a very caring, very loving person, and was daddy's little girl."
Campbell had gone to the race with her best friend Karen, whose boyfriend was running in the race, her father said.
"They wanted to take a photograph of him crossing the finish line, but the explosion went off and they were right there," he said. "It's pretty devastating.
The friend suffered a severe leg injury.
Krystle's grandmother told multiple media outlets that the family was initially told Campbell was alive because of a name mix-up.
When her father arrived at Massachusetts General Hospital, however, he learned that his daughter had died.
Krystle's grandmother, Lillian Campbell, said somewhere on the way to the hospital, their names got mixed up.
Lillian Campbell said her son was "devastated" when he found out the truth and almost passed out.
BRITTANY LORING: AN AMBITIOUS STUDENT
Brittany Loring was spending Monday, her 29th birthday, cheering on her friend in the Boston Marathon. A day later, she lay in critical condition with injuries to her head, leg and fingers.
"We've had so many calls. Everybody's just upset over it," grandmother Philomena Loring told the Lowell Sun. "I put her on the prayer line at my church."
Loring is simultaneously pursuing degrees in law and business administration at Boston College. She's also a runner, finished 80th in the Boston College MBA 5K on April 6.
The Associated Press contributed to this report.