Carlos Arredondo was in the bleachers by the finish line of the Boston Marathon when the first bomb went off directly across the street.

“Loud,” he says. “The fireball that came out. Also the smoke.”

In the next moment, the 53-year-old from Boston was vaulting a barricade and racing straight into the acrid cloud, wearing a cowboy hat like some Western hero.

“That’s the first reaction, ‘We have to go help somebody!’” he says.

He saw a big pool of blood and several severed limbs. And there was a young man in a gray top directly in front of him who was trying desperately to stand up.

“He had no legs,” Arredondo says.

A second bomb went off 100 yards away. Arredondo kept his focus on the young man, asking his name and offering his own.

“I told him, ‘My name’s Carlos, you’re going to be OK, help is on the way,’” Arredondo recalls.

The man was bleeding so heavily he seemed likely soon to be beyond help unless Arredondo applied tourniquets. He grabbed of the first thing he saw that might work.

“Somebody’s sweater,” he said. “I tore it apart.”

The tourniquet on the left leg went just above the knee, below which there was nothing but torn flesh and a length of bare bone.

Another tourniquet went on the right leg. Arredondo next grabbed a wheelchair that somebody was pushing past. He placed the young man into it and moved it as quickly as he could while still holding up what remained of the legs.

The young man was still conscious, and Arredondo told him to assist as best he could by working his hands on the wheels.

They proceeded on through the panicked crowd as quickly as circumstances allowed.

“That was a whole marathon,” Arredondo says. “Getting people out of the way and getting him help.”

Arredondo continued with the young man on into the big white medical tent that had been set up for runners who became dehydrated or hobbled during the race. The runners now watched this other kind of marathon, with Arredondo and the bleeding young man with no legs proceeding all the way through the tent to the exit where ambulances had begun to arrive.

A woman had begun pushing from the rear, and a cop had stepped in to help from the front. The young man was still conscious when he was loaded into an ambulance.

“I told him,’ You’re going to be fine,’” Arredondo says.

More ambulances and then still more arrived for the other injured, who totaled at least 100, including at least eight children, one just 2 years old. The three dead included an 8-year-old, killed at the end of the marathon whose final mile this year had been dedicated to the murdered school kids at Sandy Hook.

Arredondo remained at the scene, still wearing buttons bearing photos of his two sons that he had affixed to the chest of his sweatshirt before coming to the race. The older one, Alexander, had been just 20 when he was killed serving with the Marines in Iraq. The younger son, Brian, subsequently hanged himself.

“Next to him we find a letter about how the other brother died,” Arredondo now told me.

The marathon is on Patriots’ Day, and Arredondo had come to hand out American flags in the memory of his sons and the others who have lost their lives as result of the war on terror that was supposed to help keep us safe.

“I hand out 200!” he reported.

He had only one flag left when the bombs went off, and it had become soaked in blood as he helped the young man.

“Look at the flag, all bleeding,” Arredondo said.

He furled it and stuck it in his back pocket. He stood with more blood on the sleeve of his sweatshirt and on his pants and on his shoes as the marathoners who had filled the streets were replaced by SWAT teams in body armor and a man in a bomb disposal suit right out of the movie The Hurt Locker. Investigators on the other side of the crime scene tape searched for clues, testing for chemical traces of the explosives, collecting surveillance-camera footage.

A 20-year-old Saudi man had been running away from the explosions when a civilian tackled him and alerted a cop. But the Saudi had been cooperative and his identification had checked out, making the authorities discount early rumors that they had a suspect.

What is known about the killer or killers is that they were not just trying to make a political point by disrupting the world’s oldest marathon on the day that marked both the start of the American Revolution and the deadline for taxes. They had been trying to kill as many people as they could by detonating the bombs in the crowd.

Pure evil had struck again, as it had at the World Trade Center, as it had at Sandy Hook. But, as always, good had risen to face it, and Arredondo had not been alone. Many others had immediately pitched in to help before the smoke had fully cleared. One man from Duxbury, Massachusetts, threw himself atop his kids when the bombs detonated, and a stranger threw himself on top of the dad to give them more protection.

As night deepened, Arredondo returned home with the bloodied flag and the photos of his sons. He had gone to the marathon to honor them. And that is exactly what he had done.

Meanwhile, the search for the killers continues.