When two bombs exploded near the Boston Marathon finish line Monday, thousands of people ran for their lives and others ran straight into the chaos to help.
Carlos Arredondo and his wife were in a VIP section passing out U.S. flags to National Guard runners with Run for the Fallen Marine, an organization established to honor Marines who have been killed since the 9/11 attacks, when one of the bombs exploded right in front of them.
"My first instinct was just to run across the street and start helping people," Arredondo, 52, told "Good Morning America" today.
He sprang into action, rushing to help a bystander who had lost both of his legs. Arredondo helped control the bleeding and stayed by the man's side, holding his hand until help arrived.
A still-shaky Arredondo called it "a horrifying scene."
"He's a little bit in shock and that's why he's shaking," Arredondo's wife, Melida, 47, said.
One of the National Guard runners was in the marathon in honor of the Arredondos' son, who was killed by sniper fire in Iraq in 2004.
Monday's two explosions left at least 45 people injured and three dead, including an 8-year-old boy from nearby Dorchester, Mass. At least 17 people are in critical condition.
Medics and authorities already on-scene to care for the runners turned into first responders when the bombs exploded. For some bystanders, adrenaline kicked in and they stepped in to help.
Army veteran Bruce Mendelsohn was blocks away from the explosion and said he was knocked out of his seat by the blast. He got up and headed toward the scene.
"I was with my brother, who had just finished the marathon, and I yelled at him to get all the people back away from the windows," Mendelsohn told "Good Morning America."
Mendelsohn described seeing blood on the sidewalks and multiple people with lower body injuries.
"I tried to render any medical assistance I could," he said. "I tried to help the Boston police department who were on the scene."
COMPLETE COVERAGE: Boston Marathon Explosion
Former New England Patriots player Joe Andruzzi, who was there in support of his cancer foundation, was seen carrying an injured woman to safety.
"Our thoughts & prayers are with all the victims and their families impacted at today's Boston Marathon," Andruzzi, 37, later tweeted. "TY [thank you] to all our emergency personnel."
The stories of bravery quickly spread to social media.
One witness tweeted a photo of dozens of ambulances lining the streets with the caption, "This is what a hero looks like."
Another man tweeted, "My boss' brother was at the Boston Marathon and he said he carried a five-year-old who had lost her legs five blocks to an ambulance. #hero"
"That's what Americans do in times of crisis," Suffolk County District Attorney Daniel Conley, a Boston mayoral candidate, said. "We come together and we help one another. Moments like these, terrible as they are, don't show our weakness, they show our strength."