Out of the horror of the Boston marathon bombings, there are uplifiting stories of courage and kindness emerging.
People who ran into the aftermath of the explosions to help the wounded - people who weren't trained in medical care, as the video at the top shows.
Here are some of them.
Arredondo was in the VIP section near the finish line waiting for a runner who was competing in memory of his son Alexander Arredondo, a former U.S. marine who was killed in Iraq.
As soon as the bombs went off, he ran across the street to rip down fencing to get to people who were wounded - using his own clothes and towels to try to stop bleeding.
In the photo below, Arredondo is stopping the bleeding of a man who'd lost both his legs and was being carried away in a wheelchair.
"I kept talking to him. I kept saying: 'Stay with me, stay with me,'" Arredondo told the Portland Press Herald, a newspaper in Maine.
In the photo below, he's carrying a small, bloodied American flag - one of many he was handing out to soldiers at the marathon.
Mother Jones has video of Arredondo telling bystanders about the bombings. He's clearly shaken by what happened.
After his son was killed in Iraq, Arredondo became a peace campaigner known for wearing a cowboy hat.
With his friend Arredondo, John Mixon - a Vietnam veteran - ran across the street to help people who were hit by the explosions.
"When we got over there, it was just a pile of bodies - people with legs missing," Mixon said. "It was absolutely like a war scene. This was worse, because it was all innocent people, just defenseless. They were just lying in a pile, gunpowder all over them, burnt."
Together, Arredondo and Mixon sat in his daughter's apartment last night, trying to cope with what had happened.
"I'm in shock. We're both still in shock."
Many of the people who were in the marathon just kept running to the Massachusetts General Hospital to give blood for the wounded.
Reports of Marathon Runners that crossed finish line and continued to run to Mass General Hospital to give blood to victims #PrayforBoston— NBC Sports Network (@NBCSN) April 15, 2013
So many runners showed up, they had to be turned away and the hospital and the Red Cross actually stopped accepting donations - sending out this tweet.
But as The Telegraph writes, "the volunteers kept on coming, leading the organization to tweet an hour later: "We do *NOT* need blood at this time. Please schedule a future donation."
In fact, the Red Cross had to set up an appointment book for everyone who was lining up to give blood.
Other runners tore off their shirts to use as tourniquets to stop people's bleeding.
A spectator John Ross ripped off his belt to make a tourniquet and stop the bleeding of a badly injured person
"Somebody's leg flew by my head," he told the Boston Herald. "I gave my belt to stop the blood."
Andruzzi, a former NFL star who won the Super Bowl three times with the New England Patriots, carried victims to safety.
He was near the finish line waiting for his wife, and was at the marathon in support of his cancer foundation, which was hosting an event on Boylston Street - where the bombs went off.
Later, after receiving requests for interviews, Andruzzi put out a statement saying...
"I appreciate the interest in hearing our perspective on today's horrific events [but] the spotlight should remain firmly on the countless individuals - first responders, medics, EMTs, runners who crossed the finish line and kept on running straight to give blood, and the countless civilians who did whatever they could to save lives. They were the true heroes."
All three of Andruzzi's brothers were firefighters in New York who responded to the September 11 attacks.
Dr. Chris Rupe
A general surgeon from Salina, Kansas, Rupe (left with his wife Abbey) had just finished the race when the bombs went off. The first explosion happened about 30 seconds after he crossed the line.
At that moment, he ran to the scene and spent about an hour in a medical tent treating wounded people until most were taken to hospital.
"I'd just run 26 miles. I was starting to get tired," Rupe told The Salina Journal. "There were a lot of great people who were there. There are a lot of good people in the world."
"Doctors are doctors, but I think they were glad to have someone who knows about treating wounds," he said.
Someone started a Google document offering to open their home to anyone who needed help.
Within an hour and a half, there were nearly 3000 offers of rooms and shelter.
Here's an example of the kind of offers being made: "Located across the street from Mass General Hospital. I have a couch and an inflatable twin mattress for anyone who needs to stay."
A woman named Kristin Corona in central Boston wrote: "I have a couch to offer and two beautiful chihuahuas to love you. My apartment is open to anyone in need."
Restaurants in the area offered free meals and shelter to those caught in the tragedy.
The owner of one restaurant said "My coworkers and staff deserve a lot of credit: not one blinked when asked, not one went home when they could, those not working came in."
Other people offered rides, meals, clean clothes and showers.
Runner just told me he's been stopped numerous times by Bostonians asking if he needs a bed or a shower.— Billy Baker (@billy_baker) April 15, 2013
And there are other stories passed on through Twitter. Here's just a few.
My boss's brother was at the Boston Marathon, and he said he carried a five-year-old who had lost her legs five blocks to a ambulance #hero— Nick Murillo(@NickM516) April 15, 2013
Just heard about a doctor who running the marathon that immediately after went to his hospital to start performing surgeries. #hero— Lucas Wiman (@lwimann) April 16, 2013
Boston's 1st responders line up to help those injured.Take a look people - this is what a hero looks like. twitter.com/BosGuy/status/...— BosGuy (@BosGuy) April 15, 2013
For all the horror today, what I saw after the explosions was a city of compassion and caring, of people wanting to help. The real Boston.— Billy Baker (@billy_baker) April 16, 2013